[1.9] defined? does not yet compile, is used by RubyGems

For 1.6RC1, I disabled defined? compilation because the code is rather complex and requires numerous small changes. Unfortunately this also means that JRuby can't be run in -X+C mode, because we load rubygems at startup which uses defined in a few places.

For 1.6, we need to try to get defined? compiling. This will be painful, but it is necessary to get all 1.9 code to compile and to run with -X+C at all in 1.9 mode.