configure: remove bashism.

== is bash-only; for other shells this gives an error (meaning that you won't
get the sanity check):

	./configure
	Compiling ccan/tools/configurator/configurator...done
	./configure: 148: [: gcc: unexpected operator

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" == "clang" ]; then
+    if [ "$CC" = "clang" ]; then
 	echo "Address sanitizer (ASAN) is currently only supported with gcc"
 	exit 1
     fi