python: Remove usage of PYTHON_VERSIONS

* configure.ac: Remove PYTHON_VERSIONS subst.
* lang/python/Makefile.am: Use basename of python as builddir prefix.
* lang/python/tests/run-tests.py: Likewise.

Two variables needs be at sync PYTHONS and PYTHON_VERSIONS, these may go
out of sync in some cases, for example in Gentoo where default python is
3.4 we get:

PYTHON='/usr/bin/python2'
PYTHONS='/usr/bin/python /usr/bin/python2'
PYTHON_VERSIONS='2.7 3.4'

We can use the basename of the python interpreter to achieve similar
effect without having to sync indexes between these two variables.

• Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>