Comment # 4 on bug 1207307 from Robert Frohl

How about just shipping a additional copy of ruby.h in /usr/lib64/ruby/include
with the ruby<version> package?

This does not conflict with the devel package and would make a package from the
stdlib work out of the box, without the need to install any additional, larger
package.

I tested this with the extracted ruby.h from the ruby2.5-devel rpm, without
having the rpm installed, and that seems to fix the issue.