Make determining the compiler underlying mpif77/90 more robust

@milljm and I came across a weird situation where 'mpif90 -show' returned

gfortran ... -lmpifort ...

This caused our Makefile to improperly detect that the Fortran
compiler was *both* gfortran *and* ifort.  The fix is to only strip
off the first output of the 'mpif90 -show' command.

Refs #3707.