cmake: add CMakeLists.txt
Merge branch 'cmake' into dev/fix-glx-ext-checking

* cmake:
  cmake: add CMakeLists.txt
glxext: check for glXGetCurrentDisplayEXT

It is required in isGLXExtSupported and _GLX_EXT_import_context has a
chicken-and-egg problem, so just check for one of the functions instead.