Override impi defaults for mpicc etc.

Added a switch so that one can override the defaults of the impi installation. Defaults are to wrap the system compilers with mpicc, mpicxx, mpif90... 

The switch overrides this behaviour by setting environment variables. 

These environment variables also affect the behaviour of mpigcc and mpigxx so I've added aliases to give back the expected behaviour that they wrap the GNU compilers.
giving the parameter a generic name allows us to define the same parameter for the other MPI libraries (OpenMPI, MVAPICH, MPICH, ...), with a similar result