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.

v2: Prune remainders of swr support.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4348>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4348>

src/SConscript

@@ -45,17 +45,7 @@ if env['platform'] != 'windows':
 SConscript('mapi/glapi/gen/SConscript')
 SConscript('mapi/glapi/SConscript')
 
-# Haiku C++ libGL dispatch (renderers depend on libgl)
-if env['platform'] in ['haiku']:
-    SConscript('hgl/SConscript')
-
 SConscript('mesa/SConscript')
 
-if not env['embedded']:
-    if env['platform'] not in ('cygwin', 'darwin', 'freebsd', 'haiku', 'windows'):
-        SConscript('glx/SConscript')
-    if env['platform'] == 'haiku':
-        SConscript('egl/SConscript')
-
 SConscript('gallium/SConscript')