Mark ppx_deriving as an explicit optional dependency (fix #596).
This prevents the following scenario:
- install
why3
-> no s-expr support - install
ppx_sexp_conv
-> why3 is recompiled, but still no s-expr support - install
ppx_deriving
-> why3 is not recompiled
This bug did not exist originally, as ppx_sexp_conv
was depending on ppx_deriving
. This is no longer the case nowadays.