Use libc++ on OSX
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.
ionux/MCServer,nichwall/cuberite,nevercast/cuberite,Fighter19/cuberite,thetaeo/cuberite,thetaeo/cuberite,nevercast/cuberite,Altenius/cuberite,mmdk95/cuberite,mmdk95/cuberite,birkett/cuberite,bendl/cuberite,HelenaKitty/EbooMC,Howaner/MCServer,nicodinh/cuberite,Fighter19/cuberite,nounoursheureux/MCServer,Altenius/cuberite,zackp30/cuberite,QUSpilPrgm/cuberite,nicodinh/cuberite,thetaeo/cuberite,QUSpilPrgm/cuberite,guijun/MCServer,guijun/MCServer,jammet/MCServer,ionux/MCServer,marvinkopf/cuberite,mc-server/MCServer,electromatter/cuberite,nicodinh/cuberite,tonibm19/cuberite,HelenaKitty/EbooMC,linnemannr/MCServer,linnemannr/MCServer,marvinkopf/cuberite,nounoursheureux/MCServer,Tri125/MCServer,electromatter/cuberite,tonibm19/cuberite,mc-server/MCServer,nicodinh/cuberite,jammet/MCServer,nevercast/cuberite,Howaner/MCServer,mjssw/cuberite,Tri125/MCServer,Schwertspize/cuberite,mjssw/cuberite,Tri125/MCServer,Fighter19/cuberite,electromatter/cuberite,bendl/cuberite,ionux/MCServer,Schwertspize/cuberite,Fighter19/cuberite,Schwertspize/cuberite,tonibm19/cuberite,mmdk95/cuberite,nichwall/cuberite,Haxi52/cuberite,bendl/cuberite,birkett/MCServer,mc-server/MCServer,birkett/cuberite,QUSpilPrgm/cuberite,ionux/MCServer,electromatter/cuberite,Frownigami1/cuberite,HelenaKitty/EbooMC,birkett/MCServer,birkett/MCServer,Altenius/cuberite,Haxi52/cuberite,Howaner/MCServer,johnsoch/cuberite,mmdk95/cuberite,mc-server/MCServer,Fighter19/cuberite,nichwall/cuberite,marvinkopf/cuberite,Haxi52/cuberite,bendl/cuberite,mc-server/MCServer,birkett/cuberite,johnsoch/cuberite,zackp30/cuberite,Altenius/cuberite,kevinr/cuberite,mc-server/MCServer,Frownigami1/cuberite,tonibm19/cuberite,nevercast/cuberite,linnemannr/MCServer,linnemannr/MCServer,guijun/MCServer,nicodinh/cuberite,Altenius/cuberite,nounoursheureux/MCServer,bendl/cuberite,HelenaKitty/EbooMC,nicodinh/cuberite,kevinr/cuberite,Frownigami1/cuberite,Haxi52/cuberite,marvinkopf/cuberite,mjssw/cuberite,johnsoch/cuberite,nounoursheureux/MCServer,Schwertspize/cuberite,mjssw/cuberite,ionux/MCServer,Tri125/MCServer,mmdk95/cuberite,jammet/MCServer,birkett/MCServer,Howaner/MCServer,marvinkopf/cuberite,nichwall/cuberite,Haxi52/cuberite,kevinr/cuberite,johnsoch/cuberite,guijun/MCServer,birkett/cuberite,SamOatesPlugins/cuberite,electromatter/cuberite,tonibm19/cuberite,marvinkopf/cuberite,birkett/MCServer,nichwall/cuberite,mjssw/cuberite,guijun/MCServer,birkett/cuberite,QUSpilPrgm/cuberite,thetaeo/cuberite,jammet/MCServer,zackp30/cuberite,mjssw/cuberite,ionux/MCServer,SamOatesPlugins/cuberite,kevinr/cuberite,Frownigami1/cuberite,electromatter/cuberite,Fighter19/cuberite,zackp30/cuberite,jammet/MCServer,Tri125/MCServer,mmdk95/cuberite,guijun/MCServer,Haxi52/cuberite,nevercast/cuberite,SamOatesPlugins/cuberite,QUSpilPrgm/cuberite,birkett/cuberite,thetaeo/cuberite,kevinr/cuberite,QUSpilPrgm/cuberite,Howaner/MCServer,nounoursheureux/MCServer,nichwall/cuberite,birkett/MCServer,SamOatesPlugins/cuberite,johnsoch/cuberite,zackp30/cuberite,linnemannr/MCServer,kevinr/cuberite,tonibm19/cuberite,jammet/MCServer,Schwertspize/cuberite,HelenaKitty/EbooMC,nevercast/cuberite,Howaner/MCServer,nounoursheureux/MCServer,zackp30/cuberite,Frownigami1/cuberite,thetaeo/cuberite,linnemannr/MCServer,Tri125/MCServer,SamOatesPlugins/cuberite