Unable to add gem with C extensions to Ruby built from puppet-runtime

The Ruby built by https://github.com/puppetlabs/puppet-runtime is unable to install the ed25519 gem (even when done as part of one of the runtime builds). This appears to be an issue with how we configure Ruby.

The gem build fails to find ruby.h. This appears to happen because the include dir generated by Ruby (mkmf.rb) or the gem extension uses a cygwin path that mingw doesn't understand. That seems to be controlled by RbConfig::CONFIG['target_os'] and RbConfig::CONFIG['build_os'].

I tried patching the gem's extconf.rb to set

RbConfig::MAKEFILE_CONFIG['target_os'] = 'cygwin'

which seemed like it should disable mkmf generating the cygpath calls, but it didn't seem to work. Manually editing the generated Makefile to fix the cygwin path and re-running make seemed to work.