configure: relax --enable-address-sanitizer check a little.

If you use use CC='gcc <options>' this blocks ASAN.  Of course, no
check is perfect, but this catches the obvious misuse still.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

configure

@@ -145,7 +145,7 @@ if [ -z "$VALGRIND" ]; then
 fi
 
 if [ "$ASAN" = "1" ]; then
-    if [ "$CC" != "gcc" ] && [ "$CC" != "cc" ]; then
+    if [ "$CC" == "clang" ]; then
 	echo "Address sanitizer (ASAN) is currently only supported with gcc"
 	exit 1
     fi