Add missing HAVE_LLVM define to fix build with latest llvm

Broken since r314111

V2: pointed out by Jan Vesely
   - Use format() instead of % formating

Patch-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Signed-off-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 314261