This is fairly easy to do using the accelerate (vDSP) frameworks on Mac, as in this example:viewtopic.php?f=2&t=8297
On PC there is FFTW (but this is not free for commercial products if I remember correctly).
There is also a third party called FFTReal that might be suitable.
I have also thought it would be groovy to have a generic FFT class, and will likely grow my own one of these days if Jules doesn't get there first.
Until then, it's fairly easy to use the frameworks listed above and get them working with Juce.