configure.ac: Compute LLVM_VERSION_PATCH using llvm-config

This is the only guaranteed way get the patch level for llvm,
since the define cannot always be found in config.h depending
on the version of llvm or the build system used.

CC: 10.2 10.3 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jonathan Gray <jsg@jsg.id.au>
1 file changed