Modifying main Makefile to support C++ code

Adding CXXC and CXXFLAGS_* variables.  For now they have only been added to
the gfortran target but they can be extended to other targets in the future.
Modify landice Makefile to build dycore interface

This requires C++ compiler support in the main MPAS Makefile.  Logic has been
added to only build the interface if one of the external dycores has been
enabled.  The building of the interface C++ code has been done in a general
template for C++ code that can be extended to additional C++ files.