configure.ac: correctly manage llvm auto-detection

configure.ac: correctly manage llvm auto-detection

Earlier refactoring commits changed from one, dare I say it, broken
behaviour to another. Namely:

Before, as you explicitly --enable-gallium-llvm your selection was
ignored when llvm-config was not present/detected.
Today, the "auto" heuristics enables gallium llvm regardless if you have
llvm/llvm-config available or not.

Rework the auto-detection to attribute for llvm's presence.

v2: Set enable_gallium_llvm=no when LLVM is not found.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
Reported-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>