There are some inefficiencies in python.mk that significantly slow down
full tree scanning.  The use of bmake to obtain the current version of
a specific python is responsible for the majority of the slow done.

This commit splits out the PYTHON_PORTVERSION definition (which is the
same as the lang/python* PORTVERSION) into separate files.  With this
change, python.mk can simple include the makefile fragment instead of
spawning a new instance of make.

Different Revision:	https://reviews.freebsd.org/D4660
Approved by:		antoine (python), mva (python)