fftw3 ≤ 0.7 does not compile with OCaml 4.02.0

Fixes https://github.com/ocaml/opam-repository/pull/2514
+fftw3 v0.7.1

Fixes the compatibility with OCaml 4.02.0.
See https://github.com/ocaml/opam-repository/pull/2514
Merge pull request #2537 from Chris00/master

+fftw3 v0.7.1