build: update configure (unix) to accpept python 2.7 too

both python version 2.6, 2.7 are supported, there's little difference:
http://docs.python.org/dev/whatsnew/2.7.html#porting-to-python-2-7
enable python 2.6 / 2.7 in include header files

conditional include on _PY27 defined in configure if python is v2.7
make links for python2.7 -> python2.6 in external/ dir

this enables build with python2.7
not necessary after libs are modular