ERROR: Failed to build gem native extension.

[henrycatalinismith]

I just ran into this issue installing the ppl gem on Debian unstable, and thought it was worth documenting the solution. The full error was as follows.

$ sudo gem install ppl
Fetching: inifile-2.0.2.gem (100%)
Fetching: rugged-0.17.0.b6.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing ppl:
        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/rugged-0.17.0.b6 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/rugged-0.17.0.b6/ext/rugged/gem_make.out

The cause of this problem is a missing dependency: the ruby1.9.1-dev package. The solution is to install it and try again.

sudo apt-get install ruby1.9.1-dev
sudo gem install ppl