When running with openmp enabled, petsc must have been configured with threading

PETSc does a bunch of error checking/stack frame registration (more when compiled in debug mode).  These checks are /not/ thread safe unless PETSc has been compiled with threading enabled (`--with-threadcomm --with-pthreadclasses --with-openmp`).  This can cause hangs/segfaults at weird places in the code because memory has been overwritten in strange places.

We should probably update docs appropriately (and ask for appropriate petsc-3.4 packages?)