Handle a couple of use cases discussed in python-dev w.r.t. calculating the

Handle a couple of use cases discussed in python-dev w.r.t. calculating the
Subversion revision number.

First, in an svn export, there will be no .svn directory, so use an in-file
$Revision$ keyword string with the keyword chrome stripped off.

Also, use $(srcdir) in the Makefile.pre.in to handle the case where Python is
build outside the source tree.