ERROR: Failed to build gem native extension.

$ sudo gem install ssrf_proxy

Building native extensions.  This could take a while...

ERROR:  Error installing ssrf_proxy:
ERROR: Failed to build gem native extension.

    current directory: /var/lib/gems/2.3.0/gems/hitimes-1.2.4/ext/hitimes/c

/usr/bin/ruby2.3 -r ./siteconf20160720-30380-qhqkak.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/2.3.0/gems/hitimes-1.2.4 for inspection.

Results logged to /var/lib/gems/2.3.0/extensions/x86_64-linux/2.3.0/hitimes-1.2.4/gem_make.out

solution:
http://stackoverflow.com/questions/13767725/unable-to-install-gem-failed-to-build-gem-native-extension-cannot-load-such
accepted

There is similar questions:

`require': no such file to load -- mkmf (LoadError)

Failed to build gem native extension (mkmf (LoadError)) - Ubuntu 12.04

The solution is:

sudo apt-get install ruby-dev

Or, if that doesn't work, depending on your ruby version, run something like:

sudo apt-get install ruby1.9.1-dev

Should fix your problem.

Still not working? Try the following after installing ruby-dev:

sudo apt-get install make