automake: add SWR LLVM gen_builder.hpp workaround

automake: add SWR LLVM gen_builder.hpp workaround

As gen_builder.hpp file is generated, it contains information that is
specific to the LLVM version it originates from.

As suggested by Tim, the file seems to be forwards compatible. So in
order to produce ship a file which will work everywhere we should be
using earlies supported LLVM - 3.9.

With this we're back on track and can build all of mesa without
python/mako/flex and friends.

In the long term we might want to see if the python generators can be
updated to produce LLVM version agnostic files. At least within the
range supported by SWR.

Cc: <mesa-stable@lists.freedesktop.org>
Cc: Chuck Atkins <chuck.atkins@kitware.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>