Remove all usage of HAVE_* feature test macros in OCaml code.
Make primitives which accept () also accept a single argument in C.

Without this, OCaml and C assume different ABIs. While this may
not necessarily lead to crashes, it's still incorrect.
Get rid of all instances of "#if windows" that refer to primitives.

All "raise Not_available" calls are effectively folded into
the corresponding C stubs.
Get rid of all instances of "#if windows || android".

pa_optcomp is henceforth not needed for building.
Add Lwt_config to InternalModules of lwt.unix.
Fix deprecated Lazy.lazy_from_fun usage.
Remove deprecated lwt.extra library.
Remove pa_optcomp, as it is not used anymore.
Remove pa_lwt syntax from libraries and tests.
Make camlp4 optional.