configure.ac: increase required swr llvm to 3.9.0
configure.ac: increase required swr llvm to 3.9.0

GS implementation uses the masked.{gather,store} intrinsics,
introduced in llvm-3.9.0.  swr llvm version requirement in
automake and scons now match (scons already needed >= 3.9).

Reviewed-by: Bruce Cherniak <a5345730962d952f474b19e983398300e1cb5f29@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler