require "mkmf" ---> can't find header files for ruby?

[anne001]

I am reposting my request, in case it was not seen, buried in another
post:
the first line of extconf.rb is
require "mkmf", which is also the first line in programming ruby, ed 2
page 277.

If I just run that line, I get the same error message,
ruby
require "mkmf"
can't find header files for ruby.

I am using the ruby that comes with Tiger.
I did not find a ruby.h file, only an OSX-ruby.h which came in the
rubycocoa installation.