rubygems/rubygems — pull request #744 by voxik (2013-12-11 15:46:16+00:00)
Title: Allow to override default extension_dir using operating_system.rb

This follows the same logic as gem_dir overrides, although
default_ext_dir_for requires base_dir parameter. This allows to install
gems for example into /usr/share, but their extensions into /usr/lib64.