Improvements to FindPetsc.cmake module

Hi,

building opm-core with the petsc patches revealed some problems in the FindPetsc.cmake file - some details seemed very specific to your system. Please merge these patches.
My bad - while I think I have MPI installed (it finds the binaries too), config.h reports undef have_mpi.

There's no mpi.h in my petsc package, though. It find's the system's (openMPI) binaries and headers.

If petsc depends on it's local MPI headers then my patch is obviously wrong.