--prefix: use opt_prefix when available.
--prefix: use opt_prefix when available.

Fixes #1952.

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