I have Rails 3 and Ruby 1.9.2
Installing rmagick (2.13.1) with native extensions /opt/local/lib/ruby/site_ruby/1.9/rubygems/installer.rb:551:in `rescue in block in build_extensions': ERROR: Failed to build gem native extension. (Gem::Installer::ExtensionBuildError)
/opt/local/bin/ruby19 extconf.rb checking for Ruby version >= 1.8.5... yes checking for gcc... 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... *** 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=/opt/local/bin/ruby19 /opt/local/lib/ruby/1.9/mkmf.rb:368:in `try_do': The complier failed to generate an executable file. (RuntimeError) You have to install development tools first