python.mk: use PYVER from environment if defined

If PYVER is defined in the environment, use it as the
target Python version number.  Otherwise, discoer the
currently installed Python version for native builds.
When cross-compiling, the target Python version must
be defined or the build will be aborted.

This is then used to work out the include Python
directory as `pyhonX.Y' where X.Y is the Python
version.

Only the major and minor version numbers are used.