package/rnnoise: fix builds with BR2_OPTIMIZE_FAST

Fixes https://autobuild.buildroot.org/results/3e954cf8e4365d81e58949df50db7e82bf0d0ebf/

Rnnoise refuses to build with -Ofast unless FLOAT_APPROX is defined:

src/arch.h:171:2: error: #error Cannot build libopus with -ffast-math unless
FLOAT_APPROX is defined.  This could result in crashes on extreme (e.g.
NaN) input

So do that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/rnnoise/rnnoise.mk

@@ -12,4 +12,9 @@ RNNOISE_INSTALL_STAGING = YES
 
 RNNOISE_CONF_OPTS = --disable-examples
 
+# rnnoise refuses to build with -Ofast unless FLOAT_APPROX is defined
+ifeq ($(BR2_OPTIMIZE_FAST),y)
+RNNOISE_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -DFLOAT_APPROX"
+endif
+
 $(eval $(autotools-package))