Fix mkmf.rb can't find header files <ruby.h path>

```
ERROR: Error installing compass:
ERROR: Failed to build gem native extension.

    current directory: /var/lib/gems/2.1.0/gems/ffi-1.9.10/ext/ffi_c
/usr/bin/ruby2.1 -r ./siteconf20151118-18985-1l1bsx7.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.1.0/gems/ffi-1.9.10 for inspection.
Results logged to /var/lib/gems/2.1.0/extensions/x86_64-linux/2.1.0/ffi-1.9.10/gem_make.out
```
To Fix
`sudo apt-get install ruby2.1-dev` for 2.1.

worked 👍

Worked !

Thank you so much :) ! Saved me a lot of trouble!

sudo apt install ruby-dev will work regardless of what version is available in your repositories.

Even better:
sudo apt-get install ruby ruby-all-dev

sudo apt install ruby2.3-dev

Thanks it works for me

Even better: sudo apt-get install ruby ruby-all-dev

after hours of searching THIS WORKED TYSM can't believe this thread is like almost 10 years old. looks like it wont be fixed, hello future people in pain - april 2023

Thank you very much