Fix brew install vim --with-lua didn't install vim with lua support properly

by default, if a user has no lua installed, brew install vim --with-lua won't install lua and won't use brew-prefixed-path lua, but brew install macvim --with-lua do it(install lua if user has no lua and use brew-installed lua), so if i brew install macvim --with-lua, then brew install vim --with-lua, vim will get lua support (:version shows it), i don't mind to install a macvim, but someone may, so i made this patch .... hope it helps !