Fix linking on OS X, which does not have librt.
Fix linking on OS X, which does not have librt.

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