Since some time it seems to be somewhat impossible to install the ruby-debug gem into your rails app. The error message will most likely look similar to this one:
Building native extensions. This could take a while... ERROR: Error installing ruby-debug-base19-0.11.26.gem: ERROR: Failed to build gem native extension. /usr/local/bin/ruby19 extconf.rb checking for rb_method_entry_t.called_id in method.h... no checking for rb_control_frame_t.method_id in method.h... no *** 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.
A workaround is described on stackoverflow. On my freebsd box I had to adapt the build command for the second gem just as follows:
% sudo gem install ruby-debug-base19-0.11.26.gem -- --with-ruby-include=/usr/ports/lang/ruby19/work/ruby-1.9.3-p194
If the source is missing just run make in the ruby19 port directory.