configure.ac: Only set LLVM_LIBS if LLVM is used
configure.ac: Only set LLVM_LIBS if LLVM is used

This renames llvm_check_version_for to llvm_require_version and let it
set a variable to mark that LLVM will be used.

Use this to make a usefull configure output and to only check if the
libs are found in LLVM if it is actually used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99010

Signed-off-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler