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.

Hello!
Thank you very much for the solution, it was a 15 seconds to find this gist and have a fix.
Just a small sidenote; now it is (at least on my Linux Mint 18 Sarah):

sudo apt install ruby2.3-dev

Bye!

Thanks, worked for me too.

Thanks for sharing, that's worked for me too.

Thanks a lot! Work for me too!

👍

This didn't fix my issue I keep getting the issue and I have uploaded the screenshot of the issue.
Any suggestion for this issue.

Thanks

works to me too 👍

to me too +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