Prevent python from using system hg and svn during its installation.
Trac #15879: Prevent Python to use system hg and svn when it is built

When python gets built, the first minimal version built can try to use
the system hg and svn and fail (because its minimal or because we mess
with ld_library_path).
This can make the build process.

URL: http://trac.sagemath.org/15879
Reported by: jpflori
Ticket author(s): Jean-Pierre Flori
Reviewer(s): Volker Braun