doctor: require CLT on 10.9.
doctor: require CLT on 10.9.

Xcode-only setup is badly broken and will remain that way until fixed by
Apple. For many 10.9 issues this fixes things so let's tell the users.

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