ccanlint: don't ever used pre-built modules as dependencies.

They're often out-of-date, and we're about to do special things to the
str module compile flags, so we don't *ever* want the default.

It doesn't add much to build times, and it eliminates a nasty source
of weird errors.

tools/ccanlint/compulsory_tests/depends_build.c

@@ -53,7 +53,6 @@ static char *build_subdir_objs(struct manifest *m)
 char *build_submodule(struct manifest *m)
 {
 	char *errstr;
-	struct stat st;
 
 	if (m->compiled)
 		return NULL;
@@ -61,10 +60,6 @@ char *build_submodule(struct manifest *m)
 	if (!expect_obj_file(m))
 		return NULL;
 
-	m->compiled = talloc_asprintf(m, "%s.o", m->dir);
-	if (stat(m->compiled, &st) == 0)
-		return NULL;
-
 	if (verbose >= 2)
 		printf("  Building dependency %s\n", m->dir);