Settings for Darwin (Mac OS X) machines with Intel processors, using the
commercial Intel Fortran (ifort) compiler and GNU C++ compiler (g++).

On some Darwin systems, the clang compiler is provided as an alias to g++.
This is detected and properly supported.  On systems where both g++ and clang
exist, ensure that the PATH environement variable is set correctly such that
the desired c++ environment is used.  For example, if clang is the desired
compiler, the path to it should be prior to the path to the gcc bin directory.

Requires the following environment settings:

ESMF_COMPILER intelgcc
