fix to use more obvious Blue Gene/Q compiler wrappers
As noted in the patch description, it is not a good idea to use mpicc wrappers to XL since this is only true for some cases; users expect these to be wrappers for GCC. The use of mpixl* wrappers is unambiguous and the widely accepted practice on Blue Gene systems.
Use more obvious name of BGQ compiler MPI wrappers
While it is true that mpicc points to the XL compilers when it is the
one found in xl.ndebug/bin, this is not generally true and thus the use
of mpicc as a wrapper for XL violates the principle of least surprise.
the mpixl* wrappers are unambiguous and equally valid.