incorrect behavior when LINGUAS is set to an empty string

[hasufell]

It's common convention (e.g. in gettext) to install _all_ LINGUAS when the LINGUAS env var is _unset_.
However if the LINGUAS env var is set to an empty string, it should install none of them.
(ofc both assuming that there is no po/LINGUAS file)

However python-distutils-extra will install all LINGUAS for both cases. This way the user cannot logically disable native language support unless he sets LINGUAS="someunsupportedlanguage" which is a weird workaround.

This is especially a problem for distributors who have to do some minor hacks to fix it.