configure.ac: Set and use HAVE_GALLIUM_LLVM define

configure.ac: Set and use HAVE_GALLIUM_LLVM define

Gallium code used HAVE_LLVM to check if it needs to compile code for
LLVM in header and source files.

With the new logic HAVE_LLVM is always set. Use extra define to figure
out if LLVM is used.

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

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