Add support for petsc installs with external packages

[cekees]

On mac os x, when I build petsc with a set of external packages (like parmetis), I can't use the petsc library without setting DYLD_LIBRARY_PATH to include the petsc build artifact explicitly:

Library not loaded: /Users/cekees/proteus/hashstack/bld/petsc-n-4tj7/src/arch-darwin-c-debug/lib/libparmetis.dylib
Referenced from: /Users/cekees/proteus/hashstack/opt/petsc/4tj7/lib/libpetsc.dylib
Reason: image not found

I think the reason is because petsc links itself to it's external packages in the source directory, which gets removed later.

In the long term it seems like we may want to explicitly include petsc's external pacakges (I believe @certik mentioned that somewhere), but I've also heard the petsc team say that some of those packages need to be built with the petsc build system. Another option would be to try to get the petsc build system to use hashdist.

[certik]

Probably just rpath needs to be set? It looks like the libpetsc.dylib should contain rpath to parmetis.