COMP: compile without FFTW to avoid GPL contamination

Close #2454 (closed) . To avoid the GPL contamination inside OTB, do not compile ITK with FFTW. Instead let the fftw compilation disabled by defaut and if the user want to compile with it, inform it with a cmake message that the otb compiled relies on GNU GPL v2 license instead of Apache 2.

So by default:

• skip the fftw downloading and compilation
• do not use ITK fftw options
• do not compile and add OverlapSaveConvolutionImageFilter inside the package
• Do not compile tests relying on OverlapSaveConvolutionImageFilter

The cmake option OTB_USE_FFTW=ON should change this behavior to the actual one and emit a message explaining the license change.