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" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de>
Reported-by: Samuel Pitoiset <506e97eed81d9e59d22f4c73fb51123bb62b4a30@gmail.com>
Tested-by: Samuel Pitoiset <506e97eed81d9e59d22f4c73fb51123bb62b4a30@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler