anchor.diff:      mv ${B} ${WORKDIR}/build-fftw
   
       # configure fftwl
  +    rm -rf ${WORKDIR}/build-fftwl
   	mkdir -p ${B}
       cd ${B}
       # configure only
  
positive.code:     mv ${B} ${WORKDIR}/build-fftwl 
  
      # configure fftwf
  	mkdir -p ${B}
      cd ${B}
      # configure only
  
negative.code: 
  do_configure() {
      # configure fftw
      rm -rf ${WORKDIR}/build-fftw
  	mkdir -p ${B}
      cd ${B}
      # full (re)configure
  