build: Don't actually check for glx for glx flavour

build: Don't actually check for glx for glx flavour

GLX is now totally abstracted by Cogl so the "glx" flavour is actually
only used to determine the soname of clutter now so we don't actually
have to check for the glx header or that libGL provides any GLX symbols.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>