Error installing json Failed to build gem native extension.

jagat@nanak-P570WM:/var/www/redmine-2.4.2$ gem install json
Fetching: json-1.8.1.gem (100%)
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions into the /var/lib/gems/1.9.1 directory.
jagat@nanak-P570WM:/var/www/redmine-2.4.2$ sudo gem install json
Building native extensions.  This could take a while...
ERROR:  Error installing json:
    ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
    from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
    from extconf.rb:1:in `<main>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.1 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.8.1/ext/json/ext/generator/gem_make.out



Solution

Install ruby-dev package also

Ubuntu

sudo apt-get install ruby-dev