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.
@ocaisa: shouldn't we apply the same trick to `mpiicc`, `mpiicpc`, etc.? What if Intel MPI is installed on top of GCC? Setting `$I_MPI_CC` to `gcc` would cause `mpiicc` to produce unexpected behaviour.

If we fix it for the GCC-specific wrappers, we should also do so for the Intel specific wrappers?