Clean up CMake files

Supersedes #158
https://github.com/PCSX2/pcsx2/blob/565e72d9d787c57f06d8e1cdcb580d67aed70bd2/plugins/GSdx/stdafx.h#L171:

#ifdef ENABLE_GLES
	#include <GLES3/gl3.h>
	#include <GLES3/gl3ext.h>
#else