Some fixes for OSX compilation.  Closes #450

It is not necessary when libstdc++ is new enough (e.g. on most Linux systems).  Therefore, I felt it should only be "forced" on systems where the default (libstdc++ 4.2 on OSX) is not acceptable.  It is, of course, still possible to add it via the CMake command line or GUI if you prefer to use libc++.