Boost155

@macumber

This removes the hardcoded -stdlib=libstdc++ on OSX which was needed for boost 1.47.

The new default stdlib on OSX 10.9 is libc++, however, libc++ is included on OSX back to 10.7. On OSX <10.9, the stdlib is libstdc++.

With this change, Openstudio will use the default stdlib depending on the OSX target.

This change is also needed if Openstudio will use any c++11 features on OSX since it requires the use of libc++.

I tested with 10.9 SDK and 10.9 deployment target, 10.9 SDK and 10.8 deployment target, and 10.8 SDK and 10.8 deployment target and they all appear to compile although I'd appreciate some double checking. This was on a 10.9 machine.

@kbenne @axelstudios @nllong