r300g: share the source list

r300g: share the source list

Factor out C_SOURCES from Makefile to Makefile.sources, and let Makefile
and SConscript share it.

Note that

  $(TOP)/src/glsl/ralloc.c and
  $(TOP)/src/mesa/program/register_allocate.c

are removed from C_SOURCES in Makefile.sources and added back in
Makefile and SConscript.  The idea is that they are not part of r300g.
But having them in libr300.a makes build non-GL targets such as the
compiler tests or g3dvl much easier.  Also, for practical reason, TOP
would be an undefined variable in Makefile.sources.