I tested the code from the pull request. The gtk3 package builds. If it depends on local packages, I must have them already installed.
However, when I tried compiling my test program, it failed with lots of linker errors (see attachment). Most of them are about missing symbols which start with _imp__epoxy_. I looked into libepoxy.a, but they don't seem to be there. Actually most symbols there don't have the _imp_ prefix. I looked also into the gtk source code, but I couldn't find the corresponding function calls. In Debian documentation I read that libepoxy is an "OpenGL function pointer management library". I suppose that means that it somehow builds function names during compilation.
OpenGL support was added to gtk in version 3.16. My files build version 3.14 so I didn't have this issue.
gtk3-test.log