Document or fix `m4` installation on Mac

[palinatolmach]

On Macs, a fresh installation of kontrol build keeps throwing the following error promting to install XCode command line tools, even if m4 is installed and can be located with which m4.

What helped me and some workshop participants fix this issue is

• installing XCode application from App Store
• sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
• xcodebuild -runFirstLaunch
  (last two commands are taken from xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance nodejs/node-gyp#569)

We should either avoid this issue, if possible, or document the workaround for it.