iptables: disable static for dynamic builds

It leads to ugly runtime warnings, see:
http://patchwork.ozlabs.org/patch/386215/
It's also required for xtables-addons.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/iptables/iptables.mk

@@ -12,7 +12,9 @@ IPTABLES_DEPENDENCIES = host-pkgconf \
 	$(if $(BR2_PACKAGE_LIBNETFILTER_CONNTRACK),libnetfilter_conntrack)
 IPTABLES_LICENSE = GPLv2
 IPTABLES_LICENSE_FILES = COPYING
-IPTABLES_CONF_OPTS = --libexecdir=/usr/lib --with-kernel=$(STAGING_DIR)/usr
+# Building static causes ugly warnings on some plugins
+IPTABLES_CONF_OPTS = --libexecdir=/usr/lib --with-kernel=$(STAGING_DIR)/usr \
+	$(if $(BR2_PREFER_STATIC_LIB),,--disable-static)
 # Because of iptables-01-fix-static-link.patch
 IPTABLES_AUTORECONF = YES