Tiny changes for 4.02.0 compatibility.

Fixes #7. The bad news is that 4.02.0 compiles it 6X slower than 4.01.1
(omake -j9 takes ~25s vs. ~4s). The performance regression was fixed a couple
days after 4.02.0's release, but AFAIK there's no opam switch for 4.02.0 with
the performance fix yet.