Revert "PL/Python: Remove workaround for returning booleans in Python <2.3"

This reverts commit be0dfbad3671ed2503a2a661e70b48c5b364e069.

The previous information that Py_RETURN_TRUE and Py_RETURN_FALSE are
supported in Python 2.3 is wrong.  They require Python 2.4.  Update the
comment about that.
PL/Python: Make build on OS X more flexible

The PL/Python build on OS X was previously hardcoded to use the system
installation of Python, ignoring whatever was specified to configure.
Except that it would use the header files from configure, which could
lead to mismatches.  It was not possible to build against a custom
Python installation.

Now, we check in configure how the specified Python installation was
built and use that, supporting framework and non-framework builds.