Fix FFTW unit test with clang (grafted from 4372c66911396e2eea1d5a817c0c5dd3cfba849c)
Fix FFTW unit test with clang
(grafted from 4372c66911396e2eea1d5a817c0c5dd3cfba849c)

--HG--
branch : 3.2

robustrobotics/eigen,robustrobotics/eigen,robustrobotics/eigen,robustrobotics/eigen