revert to AC_DEFINE _PY27, fix Python2.7 not found problem

semi revert f894c147 and use AC_DEFINE _PY27, fixes NPC 345
minor change
include directly <Python.h>, no need for _PY27

turns out we've always linked to <python2.6/Python.h> (which might have caused other problems),
defining a _PY27 somehow didnt work for us, use other approach, -I correct python(VERSION) path and include
<Python.h> directly.

this solves problem on systems without python2.6
use correct python includes

turns out AC_DEFINE/AC_SUBST does not work, and we've always falled back to python2.6/Python.h
thus bugreport when compiling on a system with Python2.7 only.

As a solution we define proper python_sys_prefix/{include|lib}/python${PYTHON_VERSION} path,
and simplify files to #include <Python.h> only.
specify PYTHON_SYS_LIB too
more robust python version check, fixes NPC 349

distutils.StrictVersion had problem with '2.7.5+' which is valid version name,
Max Lapan found out using LooseVersion() fixes that.
We avoid the distutils module, use string truncation [:3] and compare.