fftw: fix build failure with old gcc

-Ofast is not available on gcc <4.6. This option is equivalent to
-O3 -ffast-math.

Fixes:
http://autobuild.buildroot.org/results/207/207a20a3ca5e62029823b1361479d5cd38fefeb0/
http://autobuild.buildroot.net/results/871/871b29c9bbf94aa05034c442f35c8771f83abe1e/



[Thomas: remove Yann SoB.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>