Ruby 1.8.5 not installing a working digest.rb on MacOSX

While trying to install a Rails app on my Mac (10.4 Tiger), I ran into
a vexing issue with digest.rb.  I installed Ruby from source on this
box (1.8.5), and have been using it daily for
weeks for Rails development and other, non-Rails Ruby playing.

Anyway, Ruby builds and installs fine, and (like I said) I have had no
other problems with my Ruby install.  But while running part of this
app's setup, I got this error:

/usr/local/lib/ruby/site_ruby/1.8/digest.rb:16:in `const_missing':
Digest class not found: Digest::SHA384 (NameError)

But it's not the app itself; it turns out that I can duplicate the
problem by running Ruby's own 'make test-all' in the build directory:

~/src/ruby rew$ make test-all
./miniruby  ./runruby.rb --extout=.ext -- "./test/runner.rb"
--basedir="./test" --runner=console
/Users/rew/src/ruby/.ext/common/digest.rb:16:in `const_missing':
Digest class not found: Digest::SHA384 (NameError)
make: *** [test-all] Error 1


Now, when I look in /usr/local/lib/ruby/site_ruby/1.8/digest.rb, I
find these lines:

require 'digest.so'

module Digest
 autoload "SHA256", "digest/sha2.so"
 autoload "SHA384", "digest/sha2.so"
 autoload "SHA512", "digest/sha2.so"
...


Now, I don't have any .so files lying around.  I believe the right
things are .bundle files on MacOSX.

I have, in /usr/local/lib/ruby/site_ruby/1.8/i686-darwin8.8.2, a
digest.bundle and a digest/sha2.bundle.  I have the same thing in
/usr/local/lib/ruby/1.8/i686-darwin8.8.2.  I don't know why they're in
both places.  They're not symlinks, and cmp(1) says they're identical
files.  Anyway...

Anyway, can someone tell me what I appear to have done wrong, or what
Ruby did wrong, when I built it to get into this situation?

By way of comparison, the hpricot gem does

 require 'hpricot_scan'

and then there is in its lib dir both 'hpricot_scan.so' and
'hpricot_scan.bundle'.

Any ideas of where to start?  A quick search for 'digest.rb' or
'digest' or 'SHA' in the Rubyforge.org Ruby bug database showed
nothing.  A friendly poster on ruby-talk suggested that it might be a
Ruby bug, and that I should bring it here.