The actual WebGL 2 distinction was done as a part of previous WebGL
cleanup. It compiles, but is completely broken, as Emscripten has some
missing bits to go through at least Context class creation.
It's meaningless to have *_INCLUDE_DIR point to the dir where
{egl.h,gl2.h,gl3.h} exists, because we're always including the files
using full path (e.g. <EGL/egl.h>). It then fails miserably at compile
time on platforms which don't have the parent directory in include path.