build system: allow python 2.6

Modified the FindPython Cmake module to also work with python 2.6, and
changed the required version in CMakeLists.txt.

Note that the current way of checking the version is not correctly working, as first the version is checked (and PYTHONLIBS_FOUND set to false in case the version is wrong), and then the library is searched (which might set PYTHONLIBS_FOUND to true independent of the version).