I ran into a problem recently on macOS Sierra, trying to install the RMagick ruby gem. I was using ImageMagick 7 installed via HomeBrew, with this error:

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

current directory: /Users/rbazinet/.rvm/gems/ruby-2.3.1@statusbox_admin/gems/rmagick-2.16.0/ext/RMagick
/Users/rbazinet/.rvm/rubies/ruby-2.3.1/bin/ruby -r ./siteconf20170203-49311-bi1omy.rb extconf.rb
checking for gcc... 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checking for outdated ImageMagick version (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
 --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/rbazinet/.rvm/rubies/ruby-2.3.1/bin/$(RUBY_BASE_NAME)

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

/Users/rbazinet/.rvm/gems/ruby-2.3.1@statusbox_admin/extensions/x86_64-darwin-15/2.3.0/rmagick-2.16.0/mkmf.log

extconf failed, exit code 1

After a bit of trying to install the gem by setting the C_INCLUDE_PATH to the location of the MagickWand.h file, the gem could still not be installed. A Google search revealed the solution. It seems the RMagick gem (v 2.16.0 as of this writing) is not compatible with ImageMagick 7.

The StackOverflow articles points outthe necessary steps to get it to work:

Install version 6 (uninstall existing imagemagick if needed):

brew install imagemagick@6

Since this is keg-only, you should then force-link it:

brew link --force imagemagick@6

This installed imagemagick version 6.9.7-4 for me.

Afterwards, the gem then installed successfully. I am including the version numbers for future readers:

This solution worked great and RMagick installed without having to jump through any other hoops. Hope this helps.