Fix clang deprecation warnings by default

[markfinal]

There are now several Xcodes with modern C++ and libc++ as standard out there.
It is time to switch the default, so that only extra work needs to be done to support older Xcodes, i.e. fix this warning

clang: warning: libstdc++ is deprecated; move to libc++ with a minimum deployment target of OS X 10.9 [-Wdeprecated]