configure.ac: Rework MESA_LLVM and LLVM detection

configure.ac: Rework MESA_LLVM and LLVM detection

Set FOUND_LLVM only when LLVM is present (checking for exact version/etc
is deferred) and use enable-gallium-llvm to indicate the global LLVM
status.

Renaming the latter is not appropriate for stable patches, so we'll
address it with a later commit.

Loosely based on work by Tobias.

v2: Check FOUND_LLVM if enable_gallium_llvm is set.

Cc: Dave Airlie <airlied@redhat.com>
Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>