Yes it is correct that for amd64 SSE is enabled by default in the compiler. But (if I'm not wrong) the FFTW --enable-sse flag does not (only) set compiler flags. Instead it activates the specially written SSE optimized kernels are compiled. Because the --enable-sse flag is not passed, those kernels are not compiled even though all amd64 processors have SSE/SSE2. Thus SSE is activated in the compiler but not for FFTW.
Yes it is correct that for amd64 SSE is enabled by default in the compiler. But (if I'm not wrong) the FFTW --enable-sse flag does not (only) set compiler flags. Instead it activates the specially written SSE optimized kernels are compiled. Because the --enable-sse flag is not passed, those kernels are not compiled even though all amd64 processors have SSE/SSE2. Thus SSE is activated in the compiler but not for FFTW.