Why aren't native extensions installed in ruby?

Question

ruby

I'm trying to install gitlab, I stumbled at the stage of installing gems.
json:

$ sudo gem install json -v '1.7.7'
Fetching: json-1.7.7.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing json:
  ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.0 extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h


Gem files will remain installed in /var/lib/gems/2.0.0/gems/json-1.7.7 for inspection.
Results logged to /var/lib/gems/2.0.0/gems/json-1.7.7/ext/json/ext/generator/gem_make.out

mysql:

$ sudo gem install mysql
Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
  ERROR: Failed to build gem native extension.

    /usr/bin/ruby2.0 extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h


Gem files will remain installed in /var/lib/gems/2.0.0/gems/mysql-2.9.1 for inspection.
Results logged to /var/lib/gems/2.0.0/gems/mysql-2.9.1/ext/mysql_api/gem_make.out

ruby from this repository: https://launchpad.net/~brightbox/+archive/ruby-ng-...

build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libreadline-dev libncurses5-dev libffi-dev curl openssh-server redis-server checkinstall libxml2-dev libxslt-dev libcurl4-openssl-dev libicu-dev logrotate

These packages are installed before building

Answer 1

Everything is clear - https://github.com/brightbox/deb-ruby2.0/issues/1