While I was trying to set up my new Mac OS X Mountain Lion installation for Ruby (on Rails) development, I encountered an issue installing the RMagick gem using the latest ImageMagick from Homebrew.

If you’re getting the following output:

checking for Ruby version >= 1.8.5... yes
extconf.rb:128: Use RbConfig instead of obsolete and deprecated Config.
checking for clang... yes
checking for Magick-config... yes
checking for ImageMagick version >= 6.4.9... yes
checking for HDRI disabled version of ImageMagick... yes
checking for stdint.h... yes
checking for sys/types.h... yes
checking for wand/MagickWand.h... yes
checking for InitializeMagick() in -lMagickCore... no
checking for InitializeMagick() in -lMagick... no
checking for InitializeMagick() in -lMagick++... no
Can't install RMagick 2.13.1. Can't find the ImageMagick library or one of the dependent libraries. Check the mkmf.log file for more detailed information.
*** 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.

After a lot of searching, this GitHub issue reveals the source of the problem (ImageMagick <-> RMagick incompatibility).

Fortunately, downgrading the Homebrew Formula for ImageMagick does the trick.

brew uninstall imagemagick
cd /usr/local/Cellar 
git checkout 834ce4a /usr/local/Library/Formula/imagemagick.rb
brew install imagemagick