Core doesn't build on 32b plateforms.

Hi,
I tried to update my installation of core (with opam) and it failed to compile because : 

> File "lib/flags.ml", line 102, characters 42-63:
> Integer literal exceeds the range of representable integers of type int
> Preprocessing error on file lib/flags.ml
> Command exited with code 2.

The fix was trivial so... here.