Log Message:
-----------
CMake: Improve checks for GTK3 on macOS
MacOS (at least with homebrew gtk3) does not have gdk/gdkx.h.
Check if the X11 (or any) back-end is used when enabling XTest/UIActionSimulator and OpenGL.
Changed paths:
M .github/workflows/ci_cmake.yml
M Makefile.in
M build/bakefiles/files.bkl
M build/cmake/files.cmake
M build/cmake/init.cmake
M build/files