MacOS.gcc_build_version: find GCC when unlinked
MacOS.gcc_build_version: find GCC when unlinked

MacOS.gcc_42_build_version was returning nil if apple-gcc42 was
installed but unlinked, which was disjoint with how other parts of
Homebrew tried to see if it was present.

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