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>

package/fftw/fftw.mk

@@ -18,7 +18,7 @@ FFTW_CONF_OPTS += $(if $(BR2_PACKAGE_FFTW_PRECISION_QUAD),--enable,--disable)-qu
 
 FFTW_CFLAGS = $(TARGET_CFLAGS)
 ifeq ($(BR2_PACKAGE_FFTW_FAST),y)
-FFTW_CFLAGS += -Ofast
+FFTW_CFLAGS += -O3 -ffast-math
 endif
 
 # x86 optimisations