Hi Nobu, I think I've managed to solve several of the issues (I have
what I hope is a fully functioning Ruby install under AIX 5.1).  Here
is what I had to do to get it to work:
export CC=cc
export CPPFLAGS=-I/usr/local/include (for self-compiled zlib.h header -
it was not being detected without this, and zlib compilation was being
skipped with no errors).

../configure --without-gcc --disable-ipv6

When configure completed:
make ruby.imp
make
Failed at socket with error above - I had to edit the ext/socket
Makefile to include -D_LINUX_SOURCE_COMPAT in the CPPFLAGS line.  The
AIX 5.1 sys/socket.h will not export the CMSG_SPACE and CMSG_LEN (and
some other) symbols unless _LINUX_SOURCE_COMPAT is defined.

After that, the build went pretty smoothly.  I was able to install
rubygems, and install several gems as well.  The only error I am seeing
now is when I run a make install-doc:
........................................
Generating RI...
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/markup/simple_markup.rb:331:in `=~':
Stack overflow in regexp matcher: /^( (RegexpError)
                        (  \[.*?\]    (?# labeled  )
                          |\S.*::     (?# note     )
                        )(?=\s|$)
                        \s*
                      )/x       from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/markup/simple_markup.rb:331:i
n `assign_types_to_lines'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/markup/simple_markup.rb:255:in
`convert'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:224:in
`markup'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:153:in
`generate_method_in
fo'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:133:in
`generate_class_inf
o'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:132:in
`generate_class_inf
o'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:90:in
`process_class'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:85:in
`generate'
        from
/tmp/wrbriggs/ruby-1.8.3/lib/rdoc/generators/ri_generator.rb:84:in
`generate'
        from /tmp/wrbriggs/ruby-1.8.3/lib/rdoc/rdoc.rb:263:in
`document'
        from ./bin/rdoc:63
make: *** [do-install-doc] Error 1

This makes me a little concerned that the build is not as error-free as
I had hoped, but I will keep you posted.  Any suggestions you have on a
cleaner way to do this build would be much appreciated.

Thank you for your response!

-Will