I'm having problems installing the RMagick rubygem on Centos 5. I've followed the steps detailed in http://rmagick.rubyforge.org/install2-linux.html but when I try:

sudo gem install rmagick

the result is:

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

/usr/local/bin/ruby extconf.rb
checking for Ruby version >= 1.8.5... yes
checking for gcc... yes
checking for Magick-config... no
Can't install RMagick 2.11.0. Can't find Magick-config in /usr/bin:/bin

*** 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=/usr/local/bin/ruby


Gem files will remain installed in /usr/local/lib/ruby/gems/1.8/gems/rmagick-2.11.0 for inspection.
Results logged to /usr/local/lib/ruby/gems/1.8/gems/rmagick-2.11.0/ext/RMagick/gem_make.out

The directory /usr/local/bin contains Magick-config but I haven't been able to get rubygems to look there. I tried the following but the result was the same:

sudo gem install rmagick -- --with-opt-dir=/usr/local/bin

Any suggestions would be appreciated.