Use AC_PATH_TOOL instead of AC_PATH_PROG for llvm-config.

Use AC_PATH_TOOL instead of AC_PATH_PROG for llvm-config.

This should help with cross-compiling and multilib when $CHOST-specific
llvm-config is expected rather than build host default one.

It will help us a bit in Gentoo where we've started using
i686-pc-linux-gnu-llvm-config for 32-bit multilib LLVM.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Michał Górny <mgorny@gentoo.org>
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=73100

CC: "10.0" <mesa-stable@lists.freedesktop.org>