math/parmetis: switch to PETSc fork

Upstream disapeared, and 2 forks are available, KarypisLab on GitHub and
the PETSc one. ATM PETSc seems more active.

Don’t install one more libmetis but depend on the regular one, and thus
don’t install the libraries under $PREFIX/lib/parmetis.

Note: manual.pdf is no more available.