gen_cxx: do not sort

As we learned, certain classes (e.g., Poller) must come first.
libneubot.json: move poller at the beginning

So that the poller appears at the beginning of neubot.hh.
gen_cxx: replace broken cast with ->pointer()

The cast was broken and crashed the Android process. Replace it with
a function named ->pointer(), which is invoked when needed.
configure: re-enable gen_cxx

Now that we have the Poller at the beginning and the ->pointer()
feature, the autogenerated neubot.hh should be OK again.
neubot.hh: regen

The net effect is, basically, that we replaced ->get_struct with
->pointer() and that the broken cast operators are gone.
Merge branch 'gen_cxx'

Fix the issues spotted by Alessandro in gen_cxx (that Poller shall
be at the beginning and that the case operator is broken) and reenable
the gen_cxx script in ./configure.