MacOS.sdk: return newest SDK if requested not found
MacOS.sdk: return newest SDK if requested not found

`MacOS.sdk` and `.sdk_path` will now return the newest installed SDK
instead of nil if called on a system that doesn’t have an SDK for the
currently-installed OS. For example, Xcode 7 on OS X 10.10 does not
include the 10.10 SDK, only the 10.11 SDK; software can be built by
specifying both SDKROOT and MACOSX_DEPLOYMENT_TARGET.

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