Return TMP to "/var/tmp" on Mac OS X

The fallback on 34b6d01b9898d730aeda0a954f81d15412a8fb8d has changed TMP to "var/tmp/tmp_dir.

This commit tries to assign TMP, on Mac OS X, as before then 5c62a3d71051e94aa5215e77461116b822dc7cc8.