fftw: minor update and major refactoring

- Unify the "single" and "float" variants, which are the same thing.
- Enable threads and openmp wrapper by default (they are very small).
- Don't use sse on i686, as I'm quite sure we have no warrant for that.
  Cf. http://en.wikipedia.org/wiki/Pentium_Pro
- Disable static libs (big, no indication we need them).
- I tested most builds using fftw (they build OK).