Add workaround for mixed Fortran / C++ code

The gfortran 4.8.1 version and corresponding environment / dependency
installed by Homebrew has a problem finding the correct libstdc++
library if C++ and Fortran code are compiled in the same library
(see Homebrew issue #20173). The gfortran formula installs its own
library at /usr/local/opt/gfortran/gfortran/lib/libstdc++.dylib which
clashes with the standard library at /usr/lib/libstdc++.dylib. This
workaround is a temporary fix until the issue is resolved.