Update header file, efftw.hpp, to accommodate various fftw dependency configurations.

[Bill Berry]

E.g., Add the following to efftw.hpp:

#if __has_include(<fftw3.h>)
#include <fftw3.h>
#endif

#if __has_include(<fftw/fftw3.h>)
#include <fftw/fftw3.h>
#endif

[Stephen Berry]

Fixed in latest commit