Use the proper python version value within a single port build

[mva]

Summary

If a port requests a different python version from the one being the
default or provided at the command line, certain directories as well as
plist entries, will use the wrong python version value (the one being
the defaullt or provided at the command line), instead of whatever it
requests.

Test Plan

# cd /usr/ports/net-p2p/deluge
# make PYTHON_VERSION=python3.4 -V_PYTHON_VERSION -VPYTHON_VERSION -Vpython_ARGS
2.7
python3.4
2
#
# make PYTHON_VERSION=python3.4 -VRUN_DEPENDS
...
/usr/local/lib/python3.4/site-packages/twisted/__init__.py:/usr/ports/devel/py-twistedCore
...

That entry is critical. py-twistedCore is only compatible with Python
2 (as stated in the port), but its PYTHON_SITELIBDIR entry gets polluted
with the user-provided PYTHON_VERSION value, although it should refer to
the (optimal) match from python.Mk.

[mva]

Committed in rP371447.

[mva]

Committed.