package/fftw: add option for fast maths

Use FFTW_CFLAGS as intermediate variable, since we're going to need it
in a later patch (about NEON optimisations).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[based on a patch by Guillaume]
Cc: guillaume william brs <guillaume.bressaix@gmail.com>
Cc: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Tested-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>