scons: Prune out unnecessary targets.

Merged Jose Fonseca requested to merge jrfonseca/mesa:scons-prune into master

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.

/cc @mareko @brianp @charmainel @bhenden

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