package/avahi: fix build without C++

Commit 02bd5eb59ff113203ea60ffee68bfdbacc4c9245 wrongly removed
avahi_cv_sys_cxx_works=yes as a result, build fails without C++

Fixes:
 - http://autobuild.buildroot.org/results/3d3596ad386144609549693c0a1aa9b704ba748c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/avahi/avahi.mk

@@ -10,7 +10,9 @@ AVAHI_LICENSE = LGPL-2.1+
 AVAHI_LICENSE_FILES = LICENSE
 AVAHI_INSTALL_STAGING = YES
 
-AVAHI_CONF_ENV = DATADIRNAME=share
+AVAHI_CONF_ENV = \
+	avahi_cv_sys_cxx_works=yes \
+	DATADIRNAME=share
 
 # Note: even if we have Gtk2 and Gtk3 support in Buildroot, we
 # explicitly disable support for them, in order to avoid the following