Installing or finding FFTW

Hi, I am quite new to ROOT and I’m really not sure how to get/link FFTW so that I can use TVirtualFFT.
At the moment when I try to run the FFT tutorial I get the error

Processing FFT.C... Error in <TMacOSXSystem::DynamicPathName>: FFTW[.so | .dll | .dylib | .sl | .dl | .a] does not exist in /Users/kxg023/alicesw/aliphysics/master/inst/lib:/Users/kxg023/alicesw/aliroot/master/inst/lib:/Users/kxg023/alicesw/aliroot/master/inst/lib/tgt_macosx64:/Users/kxg023/alicesw/alien/api/lib:/Users/kxg023/alicesw/aliphysics/master/inst/lib:/Users/kxg023/alicesw/aliroot/master/inst/lib:/Users/kxg023/alicesw/aliroot/master/inst/lib/tgt_macosx64:/Users/kxg023/alicesw/geant3/v2-0/inst/lib/tgt_macosx64:/Users/kxg023/alicesw/geant3/v2-0/inst/lib:/Users/kxg023/alicesw/geant3/v2-0/inst/lib64:/Users/kxg023/alicesw/root/alice_v5-34-30/inst/lib:/Users/kxg023/alicesw/alien/api/lib::.:/Users/kxg023/alicesw/root/alice_v5-34-30/inst/lib::/Users/kxg023/alicesw/root/alice_v5-34-30/inst/lib/cint/cint/stl Error in <TVirtualFFT::FFT>: handler not found Error: illegal pointer to class object hm 0x0 6105 FFT.C:95: *** Interpreter error recovered ***
I am using ROOT v5-34-30 and OSX, but I also have the same error using Linux.
The ‘solutions’ I have possibly found by browsing are to reinstall ROOT with some kind of instruction to install fftw (I don’t know how to do this) or to separately install FFTW (from and then somehow link it to ROOT (this is becoming a pattern, but I don’t know how to do this either).
I’ll probably need step-by-step instructions! Thanks

Thank you for the link to that post. I am still a little unsure of a few things, even after reading the link given in that post (
[li]How do I know which version of ROOT to install? Do they all have FFTW support included?[/li]
[li]Do I still need to download FFTW from and build it, or will it already be in ROOT if I choose the right version?[/li]
[li]When it says "configure ROOT with, e.g.:
–with-fftw3-incdir=“the directory where fftw3.h is”
–with-fftw3-libdir=“the directory where the fftw library is” "
does that mean I need to ./configure twice, or can I use either?