mkmf.rb can't find header files for ruby at /usr/lib/ruby/ruby.h

/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/ruby.h
Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/ffi-1.9.6 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/ffi-1.9.6/ext/ffi_c/gem_make.out

If you are using CentOS, fedora or RHEL, Run command

sudo yum install ruby-devel

If you are using debian or ubuntu, Run command

sudo apt-get install ruby-dev