title: Error when try to install ruby gtk3 gem
body: <p>I have installed ruby 2.1.6 x64 on Windows 7 x64, to C:\Ruby21-x64.
  I also downloaded and "installed" the appropriate DevKit</p>
  
  <p>When I try to install the gtk3 gem I get the following error:</p>
  
  <pre><code>C:\&gt;gem install gtk3                                                                                      
  Fetching: gobject-introspection-2.2.5-x64-mingw32.gem (100%)                                              
  Successfully installed gobject-introspection-2.2.5-x64-mingw32                                            
  Fetching: gio2-2.2.5-x64-mingw32.gem (100%)                                                               
  Successfully installed gio2-2.2.5-x64-mingw32                                                             
  Fetching: cairo-gobject-2.2.5-x64-mingw32.gem (100%)                                                      
  Successfully installed cairo-gobject-2.2.5-x64-mingw32                                                    
  Fetching: gdk3-2.2.5-x64-mingw32.gem (100%)                                                               
  Successfully installed gdk3-2.2.5-x64-mingw32                                                             
  Fetching: gtk3-2.2.5.gem (100%)                                                                           
  Temporarily enhancing PATH to include DevKit...                                                           
  Building native extensions.  This could take a while...                                                   
  ERROR:  Error installing gtk3:                                                                            
          ERROR: Failed to build gem native extension.                                                      
  
      C:/Ruby21-x64/bin/ruby.exe extconf.rb                                                                 
  checking for --enable-debug-build option... no                                                            
  checking for -Wall option to compiler... yes                                                              
  checking for -Waggregate-return option to compiler... yes                                                 
  checking for -Wcast-align option to compiler... yes                                                       
  checking for -Wextra option to compiler... yes                                                            
  checking for -Wformat=2 option to compiler... yes                                                         
  checking for -Winit-self option to compiler... yes                                                        
  checking for -Wlarger-than-65500 option to compiler... yes                                                
  checking for -Wmissing-declarations option to compiler... yes                                             
  checking for -Wmissing-format-attribute option to compiler... yes                                         
  checking for -Wmissing-include-dirs option to compiler... yes                                             
  checking for -Wmissing-noreturn option to compiler... yes                                                 
  checking for -Wmissing-prototypes option to compiler... yes                                               
  checking for -Wnested-externs option to compiler... yes                                                   
  checking for -Wold-style-definition option to compiler... yes                                             
  checking for -Wpacked option to compiler... yes                                                           
  checking for -Wp,-D_FORTIFY_SOURCE=2 option to compiler... yes                                            
  checking for -Wpointer-arith option to compiler... yes                                                    
  checking for -Wswitch-default option to compiler... yes                                                   
  checking for -Wswitch-enum option to compiler... yes                                                      
  checking for -Wundef option to compiler... yes                                                            
  checking for -Wout-of-line-declaration option to compiler... no                                           
  checking for -Wunsafe-loop-optimizations option to compiler... yes                                        
  checking for -Wwrite-strings option to compiler... yes                                                    
  checking for rb_define_alloc_func() in ruby.h... no                                                       
  checking for rb_block_proc() in ruby.h... no                                                              
  checking for new allocation framework... yes                                                              
  checking for attribute assignment... no                                                                   
  checking for rb_errinfo()... no                                                                           
  checking for cairo... yes                                                                                 
  checking for rb_cairo.h... no                                                                             
  *** extconf.rb failed ***                                                                                 
  Could not create Makefile due to some reason, probably lack of necessary                                  
  libraries and/or headers.  Check the mkmf.log file for more details.  You may                             
  need configuration options.                                                                               
  
  Provided configuration options:                                                                           
          --with-opt-dir                                                                                    
          --without-opt-dir                                                                                 
          --with-opt-include                                                                                
          --without-opt-include=${opt-dir}/include                                                          
          --with-opt-lib                                                                                    
          --without-opt-lib=${opt-dir}/lib                                                                  
          --with-make-prog                                                                                  
          --without-make-prog                                                                               
          --srcdir=.                                                                                        
          --curdir                                                                                          
          --ruby=C:/Ruby21-x64/bin/ruby                                                                     
          --enable-debug-build                                                                              
          --disable-debug-build                                                                             
          --with-pkg-config                                                                                 
          --without-pkg-config                                                                              
          --with-override-variables                                                                         
          --without-override-variables                                                                      
  
  extconf failed, exit code 1                                                                               
  
  Gem files will remain installed in C:/Ruby21-x64/lib/ruby/gems/2.1.0/gems/gtk3-2.2.5 for inspection.      
  Results logged to C:/Ruby21-x64/lib/ruby/gems/2.1.0/extensions/x64-mingw32/2.1.0/gtk3-2.2.5/gem_make.out
  </code></pre>
  
  <p>My list of installed gems is:</p>
  
  <pre><code>*** LOCAL GEMS ***                                                                                        
  
  atk (2.2.5 x64-mingw32)                                                                                   
  bigdecimal (1.2.4)                                                                                        
  cairo (1.14.1 x64-mingw32)                                                                                
  cairo-gobject (2.2.5 x64-mingw32)                                                                         
  gdk3 (2.2.5 x64-mingw32)                                                                                  
  gdk_pixbuf2 (2.2.5 x64-mingw32)                                                                           
  gio2 (2.2.5 x64-mingw32)                                                                                  
  glib2 (2.2.5 x64-mingw32)                                                                                 
  gobject-introspection (2.2.5 x64-mingw32)                                                                 
  gtk2 (2.2.5 x64-mingw32)                                                                                  
  io-console (0.4.3)                                                                                        
  json (1.8.1)                                                                                              
  minitest (4.7.5)                                                                                          
  pango (2.2.5 x64-mingw32)                                                                                 
  pkg-config (1.1.6)                                                                                        
  psych (2.0.5)                                                                                             
  rake (10.1.0)                                                                                             
  rdoc (4.1.0)                                                                                              
  require_all (1.3.2)                                                                                       
  test-unit (2.1.6.0)                                                                                       
  vrlib (1.0.16)
  </code></pre>
  
  <p>Any ideas of how to solve?</p>
  
  <p>Thanks</p>