opus-tools: disable flac when it is not selected

opus-tools expects flac since the 0.1.7 bump unless otherwise told so.
Disable it when it's not selected. Fixes:
http://autobuild.buildroot.net/results/c8c/c8c1a682336c150ee401f76b173944611b96bd4e/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/opus-tools/opus-tools.mk

@@ -19,4 +19,10 @@ else
 OPUS_TOOLS_CONF_OPT += --disable-sse
 endif
 
+ifeq ($(BR2_PACKAGE_FLAC),y)
+OPUS_TOOLS_DEPENDENCIES += flac
+else
+OPUS_TOOLS_CONF_OPT += --without-flac
+endif
+
 $(eval $(autotools-package))