configure.ac: Don't use $CLANG since it will collide with the static analyzer.

We just prefix the $CLANG environment variable in configure.ac with acv_mesa_

Found by: tinderbox
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
1 file changed