compositor: Respect WESTON_BUILD_DIR env var in weston_load_module

We were loading modules out of the system dirs unconditionally, so
tests that loaded modules would use the system ones, or fail if
they weren't installed.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-By: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

src/compositor.c

@@ -4680,16 +4680,21 @@ weston_version(int *major, int *minor, int *micro)
 WL_EXPORT void *
 weston_load_module(const char *name, const char *entrypoint)
 {
+	const char *builddir = getenv("WESTON_BUILD_DIR");
 	char path[PATH_MAX];
 	void *module, *init;
 
 	if (name == NULL)
 		return NULL;
 
-	if (name[0] != '/')
-		snprintf(path, sizeof path, "%s/%s", MODULEDIR, name);
-	else
+	if (name[0] != '/') {
+		if (builddir)
+			snprintf(path, sizeof path, "%s/.libs/%s", builddir, name);
+		else
+			snprintf(path, sizeof path, "%s/%s", MODULEDIR, name);
+	} else {
 		snprintf(path, sizeof path, "%s", name);
+	}
 
 	module = dlopen(path, RTLD_NOW | RTLD_NOLOAD);
 	if (module) {