#113 spectrogram: add FFTW3 support

[Martin Guy]

Sox specrogram has two different algorithms to do spectrograms:

lsx_safe_rdft() for dft_size of powers of two (-ysize=2^n+1) and
rdft_p(), private to spectrogram.c, which does any size but is from
150 to 250 times slower.

This adds FFTW3 support, which is about the same speed as the old lsx
routine but works at any size:

• stuff in "configure" to autodetect it and "./configure --without-fftw"
  to forcibly disable it
• code in src/spectrogram.c to use FFTW if it is enabled or the old
  routines otherwise
• changes to debian/control to build with FFTW (though the debian package
  build dies for other reasons not concerning this patch).

The output from the old algorithms and FFTW (the PNGs) are identical.

[Martin Guy]

Can close this as it is superceded by #116