Add check for gettext to brew_doctor.
Add check for gettext to brew_doctor.

If the user has linked the keg-only gettext, then a large number
of formulae which don't have "depends_on 'gettext'" will pick it
up anyway. Knowing this about the user's setup may help diagnose
bug reports.

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