configure.ac: Rename MESA_LLVM to FOUND_LLVM
configure.ac: Rename MESA_LLVM to FOUND_LLVM

This renames MESA_LLVM to FOUND_LLVM and updates the config.log report
to say if LLVM is found or not, to make clear that this does not mean
that it is used.

There are no MESA_LLVM users so drop the AC_SUBST.

v2 [Emil]
 - Polish test: -a over && test, = over ==, unquiote xyes
 - other ?

Signed-off-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler