diagnostic: make using outdated dev tools fatal
diagnostic: make using outdated dev tools fatal

A smarter form of https://github.com/Homebrew/brew/commit/559cea7fa9d7f9f2557f2523092687dec45c5316.
Travis users can't force Travis to update 10.11 to Xcode 8, so this was murdering
builds left, right & centre.

Fixes https://github.com/Homebrew/brew/issues/1096 whilst still retaining the
point of the original commit. Also offers developers an opt-out so if we need
to test something on 10.11 with Xcode 7.x we can, etc.

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