meson: automatically define `HAVE_{some}_PLATFORM` (and some cleanups along the way)
Please mind f023a995 "meson: always define HAVE_{X11,XCB}_PLATFORM
when it's enabled" as I'm not sure if this with_glx == 'dri' and with_dri_platform == 'drm'
was just a remnant of successive code changes or if there was a legitimate reason to limit the presence of these defines; as far as I can tell, these aren't even used in srx/glx/
.
Adding GLX for this reason, and tagging @dbaker @ajax @anholt who might know :)