Don't use XQuartz for e.g. :libpng on 10.8.
Don't use XQuartz for e.g. :libpng on 10.8.

XQuartz isn't added to the default library paths so if something is
linked against a Homebrew libpng then it doesn't work against the
XQuartz libpng. The CLT provides X11 on Lion so don't request users
install XQuartz if it isn't needed on Mountain Lion.

Fixes Homebrew/homebrew#14325.

konqui/brew,vitorgalvao/brew,AnastasiaSulyagina/brew,sjackman/homebrew,alyssais/brew,gordonmcshane/homebrew,pseudocody/brew,reitermarkus/brew,aw1621107/brew,vitorgalvao/brew,pseudocody/brew,nandub/brew,AnastasiaSulyagina/brew,MikeMcQuaid/brew,reelsense/brew,EricFromCanada/brew,MikeMcQuaid/brew,aw1621107/brew,sjackman/homebrew,claui/brew,konqui/brew,palxex/brew,hanxue/linuxbrew,reelsense/brew,vitorgalvao/brew,sjackman/homebrew,mahori/brew,reitermarkus/brew,DomT4/brew,EricFromCanada/brew,zmwangx/brew,zmwangx/brew,hanxue/linuxbrew,Linuxbrew/brew,tonyg/homebrew,claui/brew,aw1621107/brew,ilovezfs/brew,gordonmcshane/homebrew,jmsundar/brew,Homebrew/brew,reitermarkus/brew,toonetown/homebrew,MikeMcQuaid/brew,gregory-nisbet/brew,gregory-nisbet/brew,DomT4/brew,amar-laksh/brew_sudo,Homebrew/brew,mistydemeo/homebrew,muellermartin/dist,Linuxbrew/brew,mahori/brew,amar-laksh/brew_sudo,Homebrew/brew,nandub/brew,hanxue/linuxbrew,sjackman/homebrew,mahori/brew,tonyg/homebrew,palxex/brew,tonyg/homebrew,palxex/brew,toonetown/homebrew,ilovezfs/brew,konqui/brew,staticfloat/brew,staticfloat/brew,jmsundar/brew,maxim-belkin/brew,claui/brew,AnastasiaSulyagina/brew,EricFromCanada/brew,nandub/brew,Homebrew/brew,bfontaine/brew,nandub/brew,staticfloat/brew,rwhogg/brew,mgrimes/brew,maxim-belkin/brew,JCount/brew,reelsense/brew,muellermartin/dist,gordonmcshane/homebrew,mgrimes/brew,rwhogg/brew,DomT4/brew,mahori/brew,rwhogg/brew,muellermartin/dist,DomT4/brew,vitorgalvao/brew,JCount/brew,claui/brew,Linuxbrew/brew,mgrimes/brew,amar-laksh/brew_sudo,bfontaine/brew,konqui/brew,mistydemeo/homebrew,toonetown/homebrew,ilovezfs/brew,EricFromCanada/brew,Linuxbrew/brew,reitermarkus/brew,alyssais/brew,pseudocody/brew,JCount/brew,zmwangx/brew,maxim-belkin/brew,JCount/brew,MikeMcQuaid/brew,alyssais/brew,bfontaine/brew,mistydemeo/homebrew,gregory-nisbet/brew,jmsundar/brew