Can not use the `gettext` which is installed by brew
That formula doesn't link - you can do it by hand if you require it:


  This formula is keg-only, which means it was not symlinked into /usr/local, because macOS provides the BSD gettext library &amp; some software gets confused if both are in the library path.


The maintainers recommend:


  If you need to have this software first in your PATH run:
    echo 'export PATH="/usr/local/opt/gettext/bin:$PATH"' >> ~/.bash_profile


You can even get a more explicit warning by trying to link it manually:


  $brew link gettext. 
  
  Warning: gettext is keg-only and must be linked with --force. 
  
  Note that doing so can interfere with building software.


If you're sure you need it linked everywhere:

brew link gettext --force


See this SU thread for details https://superuser.com/questions/747324/brew-install-gettext-should-i-force-link-it