Update info for building from source

Particularly on Mac when not using homebrew (e.g. for own branches), some package versions may be too new. For example, Bison 3 does not work. Instead one needs 2.7 or so. According to @houtanb one can use

brew tap homebrew/versions
brew install homebrew/versions/bison27
brew link --force bison27

But the readme.md only states one needs Bison, version 2.5 or later, which is not correct. A similar issue may be true for Flex