The main advantage of SPDX license identifiers over the traditional
license headers is that it's more difficult to overlook inappropriate
licenses for kwin, for example GPL 3. We also don't have to copy a
lot of boilerplate text.
In order to create this change, I ran licensedigger -r -c from the
toplevel source directory.
So far it was bound to whether we build for GLES. But this is
semantically wrong. It might be possible that even on desktop gl
epoxy is built without GLX support, thus we need to reflect this.
This change ensures that epoxy/glx.h is only included if available,
that relevant code is bound to it and that checks are in place to
enforce EGL if not build with glx support.
In addtion the glxbackend.cpp is now only included in the build set
if available.
