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.
@boegel: IMHO, modifying the normal behaviour of `mpiicc` sounds kinda overzealous. 

Note that this issue relates nicely to case #100 and it is in effect a tunable implementation of the style of the very last proposition there; the bigger picture here is, that sites need control to configure those variables, in a centralized way.

btw.
The very best group of people to ask about desired behaviour of such settings are those involved with benchmarking (thinking of Jube now, but not exclusively) and mpi testing, across multiple architectures... 
So, I'd suggest to invite, via an RFC email, experts on or off the mailing list to validate the prototyped solution.