How to install rmagick

$ brew install imagemagick@6
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 1 tap (caskroom/cask).
No changes to formulae.

==> Downloading https://homebrew.bintray.com/bottles/[email protected]
Already downloaded: /Users/watson/Library/Caches/Homebrew/[email protected]
==> Pouring [email protected]
==> Caveats
This formula is keg-only, which means it was not symlinked into /usr/local.

This is an alternate version of another formula.

If you need to have this software first in your PATH run:
  echo 'export PATH="/usr/local/opt/imagemagick@6/bin:$PATH"' >> ~/.zshrc

For compilers to find this software you may need to set:
    LDFLAGS:  -L/usr/local/opt/imagemagick@6/lib
    CPPFLAGS: -I/usr/local/opt/imagemagick@6/include
For pkg-config to find this software you may need to set:
    PKG_CONFIG_PATH: /usr/local/opt/imagemagick@6/lib/pkgconfig

==> Summary
🍺  /usr/local/Cellar/imagemagick@6/6.9.8-4: 1,469 files, 22.4MB

$ LDFLAGS="-L/usr/local/opt/imagemagick@6/lib" CFLAGS="-I/usr/local/opt/imagemagick@6/include" PKG_CONFIG_PATH="/usr/local/opt/imagemagick@6/lib/pkgconfig" gem i rmagick
Building native extensions.  This could take a while...
Successfully installed rmagick-2.16.0
1 gem installed