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

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.