This is FFTW version 2.  FFTW is a collection of fast C routines to
compute the Discrete Fourier Transform in one or more dimensions.

Uptream says (but apparently there was a relaase in 2003):
If you are still using FFTW 2.x, please note that FFTW 2.x was last
updated in 1999 and it is obsolete. Please upgrade to FFTW 3.x. The
API of FFTW 3.x is incompatible with that of FFTW 2.x, for reasons of
performance and generality (see the FAQ or the manual).
Homepage:
http://www.fftw.org/
