CMakeLists: make fftw3 a requirement and not optional anymore

We have too many components in LMMS relying on FFTW3. Building LMMS
without them cripples LMMS' functionality substantially, so simply
make fftw3 a requirement.

Closes #3495736.

src/core/fft_helpers.cpp

@@ -1,7 +1,7 @@
 /*
  * fft_helpers.cpp - some functions around FFT analysis
  *
- * Copyright (c) 2008 Tobias Doerffel <tobydox/at/users.sourceforge.net>
+ * Copyright (c) 2008-2012 Tobias Doerffel <tobydox/at/users.sourceforge.net>
  * 
  * This file is part of Linux MultiMedia Studio - http://lmms.sourceforge.net
  *
@@ -25,8 +25,6 @@
 
 #include "fft_helpers.h"
 
-#ifdef LMMS_HAVE_FFTW3F
-
 #include <math.h>
 
 
@@ -243,5 +241,3 @@ float signalpower(float *timesignal, int num_values)
 	return power;	
 }
 
-#endif
-