RFC: Drop PPC support

[Sean Farley]

As discussed in the MPI thread, PPC is very old and is causing quite the
development burden. I would be in favor of discussing how much effort we
really want to spend supporting them (similar to supporting libstdc++ on
Mavericks).

We could tag some known version of the MacPorts svn tree for PPC users
and be done with it. This would allow us to concentrate on other issues.

We could also talk about how far of an OS back do we really want to
support. Tiger and Leopard are decrepit. We don't even have buildbots
for PPC nor OSes below Lion.

Should we finally drop some of this older stuff?

Comments welcomed.