ENH: respect the I_MPI_ROOT setting for INTELMPI (issue #524)

- add note in BuildIssues about the I_MPI_CC variable, which is needed
  when building with Intel-MPI and gcc/clang.

  This additional setting is needed since the changes needed to solve
  the issue of building scotch with Intel-MPI and icc (issue #434)
  means that mpiicc is now being used as the wrapper when compiling
  scotch.

- have the FOAM_MPI short name for INTELMPI start with 'impi-' instead
  of just the version number.
  Intel-MPI is often installed as /opt/intel/impi/4.1.3.049, which
  results in 'FOAM_MPI=4.1.3.049' and the mpi flavour is lost.
  Prefix these cases with 'impi-'