title: gem install sassc fails
body: <p>I am trying to install sassc
  but building the native extensions fails:</p>
  
  <pre><code>$ gem install sassc
  Building native extensions. This could take a while...
  ERROR:  Error installing sassc:
          ERROR: Failed to build gem native extension.
  
      current directory: /data/hermes/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/sassc-2.2.1/ext
  /data/hermes/.rbenv/versions/2.7.0/bin/ruby -I /data/hermes/.rbenv/versions/2.7.0/lib/ruby/2.7.0 -r 
  ./siteconf20200114-2123-fk2nkv.rb extconf.rb
  creating Makefile
  
  current directory: /data/hermes/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/sassc-2.2.1/ext
  make "DESTDIR=" clean
  
  current directory: /data/hermes/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/sassc-2.2.1/ext
  make "DESTDIR="
  compiling ./libsass/src/cencode.c
  compiling ./libsass/src/c99func.c
  compiling ./libsass/src/environment.cpp
  In file included from ./libsass/src/environment.cpp:1:0:
  ./libsass/src/sass.hpp:55:18: fatal error: string: No such file or directory
  compilation terminated.
  make: *** [environment.o] Error 1
  
  make failed, exit code 2
  
  Gem files will remain installed in 
  /data/hermes/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/sassc-2.2.1 for inspection.
  Results logged to /data/hermes/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/extensions/x86_64- 
  linux/2.7.0/sassc-2.2.1/gem_make.out
  </code></pre>
  
  <p>Obviously the compiler cannot resolve #include .</p>
  
  <p>My environment is:</p>
  
  <pre><code>$ gem --version
  3.1.2
  
  $ ruby --version
  ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]
  
  $ gcc --version
  gcc (GCC) 5.2.0
  
  $ uname -a
  Linux m4sxvmvsm6h4 2.6.32-696.20.1.el6.x86_64 #1 SMP Fri Jan 12 15:07:59 EST 2018 x86_64 x86_64 x86_64 GNU/Linux
  </code></pre>
  
  <p>Any help would be appreciated</p>