MacOS Sierra で gem install rmagick が失敗する

Ruby 向けの画像編集ライブラリ RMagick を使おうとしてハマったのでメモ。
RMagick は ImageMagick がインストールされている必要があり、その周辺でハマっている人がかなりいる模様。

発生した問題

% gem install rmagick
Building native extensions.  This could take a while...
ERROR:  Error installing rmagick:
        ERROR: Failed to build gem native extension.

    current directory: /Users/[username]/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/rmagick-2.16.0/ext/RMagick
/Users/[username]/.anyenv/envs/rbenv/versions/2.3.3/bin/ruby -r ./siteconf20170606-27346-1vveblx.rb extconf.rb
checking for clang... yes
checking for Magick-config... no
checking for pkg-config... yes
checking for outdated ImageMagick version (<= 6.4.9)... no
checking for presence of MagickWand API (ImageMagick version >= 6.9.0)... no
checking for Ruby version >= 1.8.5... yes
checking for stdint.h... yes
checking for sys/types.h... yes
checking for wand/MagickWand.h... no

Can't install RMagick 2.16.0. Can't find MagickWand.h.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/Users/[username]/.anyenv/envs/rbenv/versions/2.3.3/bin/$(RUBY_BASE_NAME)

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/[username]/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-14/2.3.0-static/rmagick-2.16.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/[username]/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/gems/rmagick-2.16.0 for inspection.
Results logged to /Users/[username]/.anyenv/envs/rbenv/versions/2.3.3/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-14/2.3.0-static/rmagick-2.16.0/gem_make.out

解決策

ImageMagick 7 with RMagick 2.16 on MacOS Sierra Can't find MagickWand.h にあった方法で解決しました。

brew uninstall imagemagick
brew install imagemagick@6
brew link --force imagemagick@6