Settings for Darwin (Mac OS X), using the GNU gfortran compiler
(part of the GCC 4.0 release) and GNU C++ compiler (g++).  Due to bugs
in early versions of gfortran, ESMF requires gfortran version 4.3 or newer.

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 gfortran
