Description:

fftw is a high performance fourier transform.
As such, archlinux could distribute it with better compilation option than the current ones.
In fact, upstream give some recommandation that we should follow (I think) :
http://www.fftw.org/doc/Installation-on-Unix.html#Installation-on-Unix

Of course one can also recompile fftw on his own system, which I gladly do with abs :-)

What I would recommand, is :
- unsetting CFLAGS : fftw will choose better compiling options.
- using --enable-sse2 for double precison and --enable-sse for single precison (this does NOT prevent it to run on non-sse2 systems !)
- using --enable-portable-binary : generates a portable binary.
- using --with-gcc-arch=arch : set the correct architecture.
(- optionnaly --enable-threads, if the other feature request  FS#18541  is accepted.)

The resulting code will still work on earlier CPUs lacking the SSE/SSE2 extensions (automatically disabled, although the FFTW library is still larger)

Additional info:
see proposed PKGBUILD.