doctor: remove gcc-4.2 warning on Xcode 4.2+
doctor: remove gcc-4.2 warning on Xcode 4.2+

Re: Homebrew/homebrew#9925, many others.

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