Revamped path access again, by Mark Hammond, to be more robust in the

light of three different situations: (1) running from build; (2)
running from installed; (3) running without being able to find an
installation (e.g. as a COM object).  The system paths in the
repository are only used for (3); the path deduced from the
installation location are used otherwise.  PYTHONHOME overrides in all
cases.

Read the comments for more details.