bustle: fix static build

Use pcap-config to list optional libpcap dependencies needed for static link.

Fixes:
http://autobuild.buildroot.net/results/569/56987b9c39376d2443b96160485f4fd8eefda9bb/

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

package/bustle/bustle.mk

@@ -10,9 +10,14 @@ BUSTLE_LICENSE = LGPLv2.1+
 BUSTLE_LICENSE_FILES = LICENSE
 BUSTLE_DEPENDENCIES = libglib2 libpcap host-pkgconf
 
+BUSTLE_PCAP_FLAGS = "-lpcap"
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+BUSTLE_PCAP_FLAGS += $(shell $(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs)
+endif
+
 define BUSTLE_BUILD_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) \
-		PCAP_FLAGS='-lpcap' -C $(@D) dist/build/bustle-pcap
+		PCAP_FLAGS="$(BUSTLE_PCAP_FLAGS)" -C $(@D) dist/build/bustle-pcap
 endef
 
 define BUSTLE_INSTALL_TARGET_CMDS