configure.ac: Require llvm-3.2 for r600g/radeonsi llvm backends

v2: Update the error message too.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66847

Cc: mesa-stable@lists.freedesktop.org
Tested-by: Marc Dietrich <marvin24@gmx.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>