scons: Prune out unnecessary targets.
This prunes out all targets except libgl-gdi, libgl-xlib, and svga, as suggested by Marek Olšák.
libgl-xlib will be remove once I have had time to confirm no automated tests we have rely upon it.
There are also a bunch of Makefile.sources which become orphaned as result, that are not taken care of in this change.