fix a few warnings on non-Linux

mostly #ifdeffing functions that are only called by #ifdeffed code

glib/gfileutils.c

@@ -1014,7 +1014,10 @@ write_to_temp_file (const gchar  *contents,
       }
   }
 #endif
+
+#ifdef BTRFS_SUPER_MAGIC
  no_fsync:
+#endif
   
   errno = 0;
   if (fclose (file) == EOF)