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.
I wouldn't say this is a bug, it's just unexpected based on the name of the wrappers...

maybe use this comment instead?

```
# force mpigcc/mpigxx to use GCC compilers, and would be expected based on their name
```

also, isn't there a `mpigfortran` too or something like that?