package/frr: add zeromq optional dependency

zeromq is an optional dependency since version 4.0 and
https://github.com/FRRouting/frr/commit/b6116506ec81735afa647af112c6aaa1ed35e9df

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/frr/frr.mk

@@ -42,6 +42,13 @@ else
 FRR_CONF_OPTS += --disable-capabilities
 endif
 
+ifeq ($(BR2_PACKAGE_ZEROMQ),y)
+FRR_DEPENDENCIES += zeromq
+FRR_CONF_OPTS += --enable-zeromq
+else
+FRR_CONF_OPTS += --disable-zeromq
+endif
+
 ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
 FRR_CONF_ENV += LIBS=-latomic
 endif