Use libc++ on OSX

Fixes issues with https://github.com/mc-server/MCServer/commit/f3dfc0349d7008efa50855c739d86918bdbf4a88 in XCode.

Tested on Mac (Makefile and XCode) and Linux.
Move to line 47.
Needs the -stdlib linker flag too.
Merge pull request #936 from archshift/patch-1

Use libc++ on OSX