configure.ac: Revert recent HAVE_LLVM changes.

configure.ac: Revert recent HAVE_LLVM changes.

This reverts changes 903eb09b5fb78d47d0f8a4bdf826a113ca2aff40..1a0aa468f354f0ee94dd383cd40ae915584624aa:

  Tobias Droste (5):
    configure.ac: Rename MESA_LLVM to FOUND_LLVM
    configure.ac: Only set LLVM_LIBS if LLVM is used
    configure.ac: Only define HAVE_LLVM if LLVM is used
    configure.ac: Set and use HAVE_GALLIUM_LLVM define
    configure.ac: Don't check LLVM version in gallium_require_llvm

They break scons build, and I'm not convinced this is the right fix.  In
particular changing HAVE_LLVM in the C code is something I'd rather
avoid no matter what.  So it's better to discuss without the pressure of
broken builds.