Use libc++ on OSX

Fixes issues with https://github.com/mc-server/MCServer/commit/f3dfc0349d7008efa50855c739d86918bdbf4a88 in XCode.

Allows use of c++11 features, and sets `__cplusplus` version number to 201103L.

Tested on Mac (Makefile and XCode) and Linux.