grep version 2.20 (and later) receive: grep: unrecognized option '--mmap'

Issue

• grep --mmap or zfgrep --mmap receive following error message after updating grep to version 2.20 (or later)

  $ grep --mmap string /dev/null
  grep: unrecognized option '--mmap'
  Usage: grep [OPTION]... PATTERN [FILE]...
  Try 'grep --help' for more information.