Let change_make_var! append to existing value.
Let change_make_var! append to existing value.

To existing flag value can be specified with "\\1".
To append values to a flag, call with:
    s.change_make_var! "THE_FLAG", "\\1 more-flag-values"

The same trick can be used to prepend or surround values as well.

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