dhcpdump: fix static build

Use pcap-config to list optional libpcap dependencies that we need to list
when building statically.

Fixes:
http://autobuild.buildroot.net/results/110/1107c21cdf656763bf7048c6c5c7899369724f5f/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dhcpdump/dhcpdump.mk

@@ -10,8 +10,13 @@ DHCPDUMP_DEPENDENCIES = libpcap
 DHCPDUMP_LICENSE = BSD-2c
 DHCPDUMP_LICENSE_FILES = LICENSE
 
+DHCPDUMP_LIBS = -lpcap
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+DHCPDUMP_LIBS += $(shell $(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs)
+endif
+
 define DHCPDUMP_BUILD_CMDS
-	$(MAKE) -C $(@D) CC="$(TARGET_CC) $(TARGET_CFLAGS)"
+	$(MAKE) -C $(@D) CC="$(TARGET_CC) $(TARGET_CFLAGS)" LIBS="$(DHCPDUMP_LIBS)"
 endef
 
 define DHCPDUMP_INSTALL_TARGET_CMDS