As always, ImageMagick is super fun after an OS upgrade. Mavericks + Ruby 2.x + RMagick failed with something very confusing today. It couldn’t find the Xext library, which is part of X11. Turns out, that was removed from Mavericks according to https://support.apple.com/kb/HT5293. Install XQuartz from https://xquartz.macosforge.org/landing and the problem goes away.
$ cat ~/.rvm/gems/ruby-2.0.0-p353@gravity/gems/rmagick-2.13.2/ext/RMagick/mkmf.log
checking for Ruby version >= 1.8.5... -------------------- yes
find_executable: checking for /usr/bin/clang... -------------------- yes
find_executable: checking for Magick-config... -------------------- yes
checking for ImageMagick version >= 6.4.9... -------------------- yes
checking for HDRI disabled version of ImageMagick... -------------------- yes
"/usr/bin/clang -o conftest
-I~/.rvm/rubies/ruby-2.0.0-p353/include/ruby-2.0.0/x86_64-darwin13.0.0
-I~/.rvm/rubies/ruby-2.0.0-p353/include/ruby-2.0.0/ruby/backward
-I~/.rvm/rubies/ruby-2.0.0-p353/include/ruby-2.0.0
-I.
-I/usr/local/Cellar/imagemagick/6.7.1-1/include/ImageMagick
-I/usr/local/Cellar/imagemagick/6.7.1-1/include/ImageMagick
conftest.c
-L.
-L~/.rvm/rubies/ruby-2.0.0-p353/lib
-L/usr/local/opt/libyaml/lib
-L/usr/local/opt/readline/lib
-L/usr/local/opt/libksba/lib
-L/usr/local/opt/openssl/lib
-L/usr/local/Cellar/imagemagick/6.7.1-1/lib
-L/usr/X11/lib
-L/usr/local/Cellar/imagemagick/6.7.1-1/lib
-lMagickCore -llcms -ltiff -lfreetype -ljpeg -lXext
-lSM -lICE -lX11 -lXt -lbz2 -lz -lm -lpthread -lltdl
-lruby.2.0.0 -lpthread -ldl -lobjc "
ld: library not found for -lXext
clang: error: linker command failed with exit code 1 (use -v to see invocation)