Background:

Currently ruby includes several useful files in its "include"
subdirectory, for extensions to use.

This is not, however, enough for certain extensions.
Ex: ruby-debug requires several other files in order to build on 1.9

node.h
vm_core.h
iseq.h
eval_intern.h
insns.inc
insns_info.inc

[at least currently].

A few other gems also require files not included [ruby-internals,
RubyNode for 1.8, etc.]

This makes it inconvenient to install certain gems.

Suggestion [from Mark Moseley]

Why not just install all the *.h and *.inc files? It's not like they
take up a lot of space.

Thoughts welcome.
=r