issue with why.2.33 configure's patches

```opam install why``` randomly fails at configuration stage. From the diff between successful and failed runs, it seems that in the latter case, the patches of ```configure.in``` in ```opam.patch.in``` are not present in ```configure```. It's probably due to the fact that ```configure.Fluorine-version``` (applied later) patches both files and depending on the resulting timestamps, ```autoconf``` may consider ```configure``` as newer than ```configure.in```. Removing ```configure``` before launching ```autoconf```, as in the proposed patch, seems to stabilize the build.