Enable setting PYTHONHOME to support applications that distribute Python

[ndjensen]

There's been a request to set PYTHONHOME for an application that distributes Python with the application. The Python C-API also supports this:

• https://docs.python.org/2/c-api/init.html#c.Py_SetPythonHome
• https://docs.python.org/3/c-api/init.html#c.Py_SetPythonHome

There's an open question of the route to take to set it: if we should use the C-API call Py_SetPythonHome(arg) or use Java's System.loadLibrary("python") or System.load(filename).

See https://groups.google.com/forum/#!topic/jep-project/9TOYn3ElbFw for more details.

[bsteffensmeier]

This has been implemented in version 3.8 with PyConfig.setPythonHome as well as the LibraryLocator which attempts to load libjep and libpython from the PYTHONHOME