MacOS.sdk_path: make sure xcode-select is sane
MacOS.sdk_path: make sure xcode-select is sane

According to a user on IRC with a brand-new Mac, xcode-select is preconfigured
to /Developer out of the box even though that doesn't exist on modern Xcode.
So we shouldn't trust that it makes any sense.
jmsundar/brew,DomT4/brew,sjackman/homebrew,DomT4/brew,konqui/brew,pseudocody/brew,mistydemeo/homebrew,Linuxbrew/brew,maxim-belkin/brew,vitorgalvao/brew,jmsundar/brew,claui/brew,mahori/brew,JCount/brew,AnastasiaSulyagina/brew,JCount/brew,Linuxbrew/brew,MikeMcQuaid/brew,muellermartin/dist,zmwangx/brew,palxex/brew,mahori/brew,zmwangx/brew,reitermarkus/brew,AnastasiaSulyagina/brew,sjackman/homebrew,aw1621107/brew,MikeMcQuaid/brew,reitermarkus/brew,bfontaine/brew,rwhogg/brew,MikeMcQuaid/brew,muellermartin/dist,mgrimes/brew,mgrimes/brew,MikeMcQuaid/brew,nandub/brew,vitorgalvao/brew,hanxue/linuxbrew,tonyg/homebrew,gregory-nisbet/brew,mgrimes/brew,DomT4/brew,vitorgalvao/brew,ilovezfs/brew,sjackman/homebrew,konqui/brew,mistydemeo/homebrew,tonyg/homebrew,konqui/brew,maxim-belkin/brew,DomT4/brew,amar-laksh/brew_sudo,amar-laksh/brew_sudo,gregory-nisbet/brew,mahori/brew,reitermarkus/brew,jmsundar/brew,mahori/brew,reelsense/brew,toonetown/homebrew,gordonmcshane/homebrew,toonetown/homebrew,JCount/brew,vitorgalvao/brew,reitermarkus/brew,amar-laksh/brew_sudo,nandub/brew,claui/brew,rwhogg/brew,mistydemeo/homebrew,EricFromCanada/brew,pseudocody/brew,Homebrew/brew,EricFromCanada/brew,Homebrew/brew,gordonmcshane/homebrew,toonetown/homebrew,JCount/brew,bfontaine/brew,palxex/brew,ilovezfs/brew,Linuxbrew/brew,reelsense/brew,maxim-belkin/brew,nandub/brew,claui/brew,aw1621107/brew,aw1621107/brew,gordonmcshane/homebrew,staticfloat/brew,pseudocody/brew,sjackman/homebrew,Homebrew/brew,rwhogg/brew,gregory-nisbet/brew,EricFromCanada/brew,hanxue/linuxbrew,tonyg/homebrew,alyssais/brew,ilovezfs/brew,alyssais/brew,palxex/brew,Linuxbrew/brew,bfontaine/brew,reelsense/brew,claui/brew,AnastasiaSulyagina/brew,nandub/brew,hanxue/linuxbrew,konqui/brew,staticfloat/brew,alyssais/brew,Homebrew/brew,muellermartin/dist,EricFromCanada/brew,staticfloat/brew,zmwangx/brew