[petsc-dev] BuildSystem update

[Dmitry Karpeev]

I've added GNUPackage(config.package.Package), which is supposed to
simplify writing configure modules
for "GNU packages" (i.e., those that are installed using the
"configure; make; make install" paradigm).
NetCDF.py has been expanded a bit to allow for a download and derives
from GNUPackage now.
Bugs are likely, of course, so if anybody has any trouble configuring
with NetCDF, let me know and I'll take
care of it.

Thanks.
Dmitry.