[petsc-dev] configure using stale packages

[Jed Brown]

If we configure with --download-pnetcdf (version 1.9.0), then update the
PETSc repository to use a new version (1.11.2), then re-run ./configure
--download-pnetcdf, we get a warning making us look like dolts:

===============================================================================                                                                                                                
      Warning: Using version 1.9.0 of package pnetcdf; PETSc is tested with 1.11                                                                                                               
      Suggest using --download-pnetcdf for a compatible pnetcdf                                                                                                                                
===============================================================================

It looks like we have a mechanism for gitcommit versions, but not for
normal numbered versions?

NB: There are other changes in updating pnetcdf that will bypass this
issue today, but I wanted to raise it in case someone had a vision for
how the logic should be organized.  It's currently spread out, and
any attempt at a fresh download is skipped if there is a matching
download directory:

    if not self.packageDir: self.packageDir = self.downLoad()