Don't compile the FFT code if FFTW is enabled

src/fft.cpp

@@ -36,6 +36,7 @@
 
 #include "fft.h"
 
+#ifndef WITH_FFTW3
 #include <cmath>
 
 void FFT::DoTransform (size_t n_samples,float *input,float *output_r,float *output_i,bool inverse) {
@@ -166,3 +167,4 @@ float FFT::FrequencyAtIndex (unsigned int baseFreq, unsigned int n_samples, unsi
 		return (-(float)(n_samples-index) / (float)n_samples * baseFreq);
 	}
 }
+#endif

src/fft.h

@@ -27,11 +27,6 @@
 //
 // Aegisub Project http://www.aegisub.org/
 
-/// @file fft.h
-/// @see fft.cpp
-/// @ingroup utility
-///
-
 #include <cstdlib>
 
 class FFT {