compling without FFTW is broken

Either with ./configure --without-fftw or cmake -DBOUT_USE_FFTW=OFF

fft_fftw.cxx: In function ‘unsigned int bout::fft::get_measurement_flag(FFT_MEASUREMENT_FLAG)’:
fft_fftw.cxx:74:14: error: ‘FFTW_ESTIMATE’ was not declared in this scope
   74 |       return FFTW_ESTIMATE;
      |              ^~~~~~~~~~~~~
fft_fftw.cxx:76:14: error: ‘FFTW_MEASURE’ was not declared in this scope
   76 |       return FFTW_MEASURE;
      |              ^~~~~~~~~~~~
fft_fftw.cxx:78:14: error: ‘FFTW_EXHAUSTIVE’ was not declared in this scope
   78 |       return FFTW_EXHAUSTIVE;
      |              ^~~~~~~~~~~~~~~