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.

include/fft_helpers.h

@@ -1,7 +1,7 @@
 /*
  * fft_helpers.h - 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
  *
@@ -29,8 +29,6 @@
 #include "lmmsconfig.h"
 #include "export.h"
 
-#ifdef LMMS_HAVE_FFTW3F
-
 #include <fftw3.h>
 
 const int FFT_BUFFER_SIZE = 2048;
@@ -85,5 +83,3 @@ int EXPORT calc13octaveband31( float * _absspec_buffer, float * _subbands,
 float EXPORT signalpower(float *timesignal, int num_values);
 
 #endif
-
-#endif