Makefile.inc: Detect if a working clang binary exists before set

Let us not assume the 'clang' binary exists and is working just because
the user selected it in .config

Change-Id: Iad3cbf4a7cda0e1c4d435fbe426b7247233973ea
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6141
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>