OK, I'm convinced.  Changing PYTHONHOME is bad.  But I am all in favor
R. David Murray's solution of extending to include something like
PYTHONHOMELIB.

Martin v. Löwis: What I want is simply to be able to use --libdir to
tell ./configure where to put standard Python libs, and I want my Python
installation to recognize this lib location without having to hack
PYTHONPATH or PYTHONHOME or anything else.

This would make the behavior of --libdir exactly what people generally
expect from using configure scripts supplied by many other projects.  As
of now, setting --libdir doesn't even do anything, silently!  But even
fixing --libdir per patch in http://bugs.python.org/issue858809 still
doesn't work because the lib path has a hardcoded component.

I just think that a sys admin, when he/she sees a configure script,
should be able to tell it what to do via flags, run it, and expect it to
Do The Right Thing, without worrying about the Python-specific
workarounds to make libs and bins go to the desired place.