First cut at adding PFFFT to the timing program.  Compiles but is
otherwise untested.
Make PFFFT test work. Only orders >= 4 are supported, but I only did
spot checks for forward FFTs of order 4, 5, and 6.
Oops. Actually use the min order.
Add more options

o -O to run only the float FFTs
o -t to run all the timing tests for just one FFT type.
Move implementation of KISSFFT to fft_time_kissfft.c.
Move implementation of Ne10 to fft_time_ne10.c.
Move implementation of FFMPEG to fft_time_ffmpeg.c.
Oops. Remove ffmpeg implementation from here.
Move implementation of CkFFT to fft_time_ckfft.c.
Oops. Remove ckfft implementation from here.
Move implementation of PFFFT to fft_time_pffft.c.
Add header files for the additional individual fft tests.
Move GetUserTime and TimeDifference to test_util.c. Update files
accordiningly.
Implement tests for CkFFT real FFT. Untested.
Allow testing of CkFFT for real FFTs. Untested.
Fix incorrect print string.
Move GenerateRealFloatSignal to gensig.c and update callers
appropriately.
Simplify test_mode code path.
Remove debugging print.
Exit if -T is given without -f.