lang/python{27,33,34}: Backport fix for upstream Issue #21166

Backport fix for upstream Issue #21166:

Prevent possible segfaults and other random failures of python
--generate-posix-vars in pybuilddir.txt build target by ensuring
that pybuilddir.txt is always regenerated when configure is run and
that the newly built skeleton python does not inadvertently import
modules from previously installed instances. [1]

This changeset has been committed for release in 2.7.9, 3.4.2, and 3.5.0.

A HUGE thank you to Ned Deily from the Python Core Development Team
for helping to identify the underlying cause, produce a fix and
wonderfully document the explanation.

[1] http://bugs.python.org/issue21166


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@365708 35697150-7ecd-e111-bb59-0022644237b5