title: CMake error with GTKmm - No package 'gtkmm-3.0' found
body: <p>This <strong><em>may</em></strong> be related to my <a href="https://stackoverflow.com/questions/48620635/gtkmm-simple-example-fails-to-compile-gtkmm-h-no-such-file-or-directory?noredirect=1#comment84242617_48620635">previous question</a>.</p>
  
  <p>I copied the code from the "Simple Example" <a href="https://developer.gnome.org/gtkmm-tutorial/stable/sec-basics-simple-example.html.en" rel="nofollow noreferrer">here</a>.</p>
  
  <p>I copied the cmake instructions from <a href="https://wiki.gnome.org/Projects/gtkmm/UsingCMake" rel="nofollow noreferrer">here</a>.</p>
  
  <p>The error I am getting is:</p>
  
  <pre><code>-- Checking for module 'gtkmm-3.0'
  --   No package 'gtkmm-3.0' found
  CMake Warning (dev) in CMakeLists.txt:
    No cmake_minimum_required command is present.  A line of code such as
  
      cmake_minimum_required(VERSION 3.10)
  
    should be added at the top of the file.  The version specified may be lower
    if you wish to support older CMake versions for this project.  For more
    information run "cmake --help-policy CMP0000".
  This warning is for project developers.  Use -Wno-dev to suppress it.
  
  -- Configuring done
  -- Generating done
  -- Build files have been written to: /path-to-files/
  </code></pre>
  
  <p>This is my CMakeLists.txt</p>
  
  <pre><code>project(SimpleExample)
  
  find_package(PkgConfig REQUIRED)
  
  set(CMAKE_PREFIX_PATH "/usr/lib/x86_64-linux-gnu/pkgconfig;/usr/share/pkgconfig")
  
  pkg_check_modules(GTKMM gtkmm-3.0)
  
  link_directories(${GTKMM_LIBRARY_DIRS})
  include_directories(${GTKMM_INCLUDE_DIRS})
  
  add_executable(SimpleExample SimpleExample.cpp)
  
  target_link_libraries(SimpleExample ${GTKMM_LIBRARIES})
  </code></pre>
  
  <p>There's a really dumb line in here <code>set(CMAKE_PREFIX_PATH "/usr/lib/x86_64-linux-gnu/pkgconfig;/usr/share/pkgconfig")</code> which I'm fairly sure isn't supposed to be there, however in my previous question I was struggeling to figure out why <code>pkg-config</code> wasn't finding the correct locations for the <code>gtkmm-3.0.pc</code> file.</p>
  
  <p>So I did a search to try and figure out how to set the paths for <code>pkg-config</code> manually in a CMake file. That is what I came up with. However it does not work.</p>