Question : Error when installing Mongrel Gem - ERROR: Failed to build gem native extension.

dallasray@justis:~$ sudo gem install mongrel mongrel_rails
Select which gem to install for your platform (i486-linux)
 1. mongrel 1.1.3 (java)
 2. mongrel 1.1.3 (i386-mswin32)
 3. mongrel 1.1.3 (ruby)
 4. mongrel 1.1.2 (ruby)
 5. mongrel 1.1.2 (mswin32)
 6. mongrel 1.1.2 (java)
 7. Skip this gem
 8. Cancel installation
> 4
Install required dependency fastthread? [Yn]  y
Select which gem to install for your platform (i486-linux)
 1. fastthread 1.0.1 (mswin32)
 2. fastthread 1.0.1 (ruby)
 3. Skip this gem
 4. Cancel installation
> 2
Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Gem::Installer::ExtensionBuildError)
    ERROR: Failed to build gem native extension.

ruby extconf.rb install mongrel mongrel_rails
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:1


Gem files will remain installed in /var/lib/gems/1.8/gems/fastthread-1.0.1 for inspection.
Results logged to /var/lib/gems/1.8/gems/fastthread-1.0.1/ext/fastthread/gem_make.out
dallasray@justis:~$

Answer : Error when installing Mongrel Gem - ERROR: Failed to build gem native extension.

You need to install the libc6-dev package.

On debian-based systems that's "sudo apt-get install libc6-dev"