builder: builddir != srcdir was broken becasue we never created the buildir

builder/builder-module.c

@@ -654,7 +654,11 @@ builder_module_build (BuilderModule *self,
 
       if (use_builddir)
         {
-          build_dir = g_file_get_child (source_dir, "_build");
+          build_dir = g_file_get_child (source_dir, "_xdg_app_build");
+
+          if (!g_file_make_directory (build_dir, NULL, error))
+            return FALSE;
+
           if (self->cmake)
             {
               configure_cmd = "cmake";