configure.ac: Only define HAVE_LLVM if LLVM is used

Make sure that HAVE_LLVM compiler define is only set if LLVM is
actually used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99010
Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Tobias Droste <tdroste@gmx.de>
v2 [Emil] fold within the existing conditional
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

(cherry picked from commit 143c566a81fa7425df64ca83b8131e016af1d660)
1 file changed