setup: attempt to work-around the problem that paths might end with trailing back...

setup: attempt to work-around the problem that paths might end with trailing back-slashes (on Windows) by appending a PATHSEP (i.e. ":" or ";") instead of an OSSEP (i.e. "/" or "\")

I don't know what will happen if the path ends up with something like
"C:\Programs and Files\Whatever\;" on Windows, and then that gets passed to
cygwin bash.  This reminds me of Brian's suggestion to use Python helper
scripts (c.f. misc/find-dep-eggs.py) instead of writing this stuff in the
GNUmake language.  And *that* reminds me of the idea of writing the whole damn
thing in Python instead of in GNUmake, i.e. make all of our build tools be
plugins for setuptools instead of being GNUmake targets.