configure.ac: honour LLVM_LIBDIR when linking against LLVM

configure.ac: honour LLVM_LIBDIR when linking against LLVM

Currently if one uses a non-default prefix, the path won't get
propagated and we'll fail at link-time.

A very quick and easy example is to install to /usr/local.
At this point, llvm-config will be picked even without the
--with-llvm-prefix, but regardless of the latter linking will fail.

Currently people can workaround that via LD_LIBRARY_PATH.

Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>
Cc: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>