Apple already provides bison on the system. However, the reason why bison is required to be installed from homebrew is that Apple provides an outdated package of Bison.
Since macOS already provides this software and installing another version in parallel can cause all kinds of trouble, the bison package we installed with Homebrew is not symlinked and hence not visible to our configuration script.
To fix that, we need to add bison to the path, ie, run
$ export PATH=/usr/local/opt/bison/bin:$PATH