Bug#734675: fftw3: Fix configury for neon support on arm64 (armv8)

[Julian Taylor]

what is the exit code if you run the command by itself? maybe libtool
is eating the error message

also try running the line without libtool, just copy the part after
gcc-4.9 dropping all the bash .../libtool --tag stuff

also the build allows parallel compilation, maybe try with
DEB_BUILD_OPTIONS="parallel=1"