A hack to augment sys.path with the build/lib.<platform> directory

created by Andrew's setup.py script, *if* we're actually running from
the build directory.  (The test for that: whether the sys.path[-1]
ends in "/Modules".)

This has one disadvantage: it imports a fair amount of code from the
distutils package, just in order to be able to calculate the correct
pathname.  See if I care. :-)