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).

pkgs/applications/graphics/gimp/plugins/default.nix

@@ -146,10 +146,9 @@ rec {
   gmic =
   let
     imagemagick = pkgs.imagemagickBig; # maybe the non big version is enough?
-    fftw = pkgs.fftw.override {pthreads = true;};
   in pluginDerivation rec {
       name = "gmic-1.5.7.2";
-      buildInputs = [imagemagick pkgconfig fftw gimp] ++ gimp.nativeBuildInputs;
+      buildInputs = [imagemagick pkgconfig pkgs.fftw gimp] ++ gimp.nativeBuildInputs;
       src = fetchurl {
         url = mirror://sourceforge/gmic/gmic_1.5.7.2.tar.gz;
         sha256 = "1cpbxb3p2c8bcv2cbr150whapzjc7w09i3jza0z9x3xj8c0vdyv1";