handling of src paths to files in ports/

[Roland McGrath]

I'm certainly fine with better formalizing the interface between configure
and the various fragments.  It should include some clear comments somewhere
listing the variables that are intended to be interface rather than local
implementation detail.  It's unwise to use overly-generic names such as
srcdir and builddir, which have canonical (srcdir) or presumable (builddir)
global meanings already.


Thanks,
Roland