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>

package/fftw/Config.in

@@ -49,4 +49,17 @@ config BR2_PACKAGE_FFTW_PRECISION_QUAD
 
 endchoice
 
+config BR2_PACKAGE_FFTW_FAST
+	bool "optimise for speed over accuracy"
+	help
+	  Optimise for fast math functions, at the expense of accuracy.
+
+	  Say 'y' if you need speed and can live with inaccuracies in
+	  the results. Say 'n' (the default) if accuracy is of utmost
+	  importance.
+
+	  This basically uses gcc's -Ofast optimisation level, which in
+	  turn is basically using gcc's -ffast-math. See the gcc manual
+	  for what this means.
+
 endif

package/fftw/fftw.mk

@@ -14,4 +14,11 @@ FFTW_CONF_OPTS += $(if $(BR2_PACKAGE_FFTW_PRECISION_SINGLE),--enable,--disable)-
 FFTW_CONF_OPTS += $(if $(BR2_PACKAGE_FFTW_PRECISION_LONG_DOUBLE),--enable,--disable)-long-double
 FFTW_CONF_OPTS += $(if $(BR2_PACKAGE_FFTW_PRECISION_QUAD),--enable,--disable)-quad-precision
 
+FFTW_CFLAGS = $(TARGET_CFLAGS)
+ifeq ($(BR2_PACKAGE_FFTW_FAST),y)
+FFTW_CFLAGS += -Ofast
+endif
+
+FFTW_CONF_OPTS += CFLAGS="$(FFTW_CFLAGS)"
+
 $(eval $(autotools-package))