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.
Thanks for your suggestion.

I thinks the linking problem dues to some setting in OPM cmake system. I need to figure it out. 

It's true if you use serial PETSc, then you need to link to its own ``mpi.h``, which locates in ``${PETSC_ROOT}/include/mpiuni/``

I don't if build opm with MPI can solve this problem, but it will be appreciate  if you can solve this liking problem.:)