When testing under the ruby/ruby setup, mkmf.rb needs to the `$extout`
global variable set properly.

This is because, in this particular case, the `ruby.h` header needed to
compile extensions is constructed from
`$(extout)/include($arch)/ruby/config.h` but `$extout` is not set by
default.

I tried to fix this in mkmf.rb itself but I couldn't figure it. But
setting it externally to workaround the issue fixes the specs, so I'll
start with that. Also setting it externally causes issues when running
specs upstream against Ruby 2.3 (I guess because of some difference with
Ruby 2.3 mkmf.rb implementation). So I'm avoiding doing it on Ruby 2.3 to
woraround that.

https://github.com/rubygems/rubygems/commit/d782984585