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.
Did you build OPM with MPI  or not? If not, when you delete this line  ``PATHS ${PETSC_ROOT}/include``, FindPetsc will find the system "mpi.h", which will break you build process. To be honest, I am not a expert of cmake, this routine works fine for me several months ago, but now it has some problems, it seems that even you find ``${PETSC_ROOT}/include/mpiuni/mpi.h``, the compiler will not link it. I need to figure it out.
Indeed, I applied your patch, due to the mpi.h problem , it broke my build process.