xl2tp: fix static build

Use pcap-config to list dependencies of libpcap.

Fixes:
http://autobuild.buildroot.net/results/87f/87ff92831d265d437aa0dfcfb99e954f76597761/
http://autobuild.buildroot.net/results/cd5/cd5487e76259752cbe58b171f9a58970916f7bb8/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/xl2tp/xl2tp.mk

@@ -10,9 +10,13 @@ XL2TP_DEPENDENCIES = libpcap openssl
 XL2TP_LICENSE = GPLv2
 XL2TP_LICENSE_FILES = LICENSE
 
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+XL2TP_LDLIBS = LDLIBS="$(shell $(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs)"
+endif
+
 define XL2TP_BUILD_CMDS
 	$(SED) 's/ -O2 //' $(@D)/Makefile
-	$(TARGET_CONFIGURE_OPTS) $(MAKE) -C $(@D)
+	$(TARGET_CONFIGURE_OPTS) $(MAKE) $(XL2TP_LDLIBS) -C $(@D)
 endef
 
 define XL2TP_INSTALL_TARGET_CMDS