configure: Use pkg-config to detect libiptc used by connmark/forecast

This ensures the library is available.  On Debian/Ubuntu it is a dynamic
library provided by the iptables-dev package.

src/libcharon/plugins/connmark/Makefile.am

@@ -4,7 +4,7 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir)/src/libcharon
 
 AM_CFLAGS = \
-	$(PLUGIN_CFLAGS)
+	$(PLUGIN_CFLAGS) $(libiptc_CFLAGS)
 
 if MONOLITHIC
 noinst_LTLIBRARIES = libstrongswan-connmark.la
@@ -17,4 +17,4 @@ libstrongswan_connmark_la_SOURCES = \
 	connmark_plugin.h connmark_plugin.c
 
 libstrongswan_connmark_la_LDFLAGS = -module -avoid-version
-libstrongswan_connmark_la_LIBADD = -lip4tc
+libstrongswan_connmark_la_LIBADD = $(libiptc_LIBS)

src/libcharon/plugins/forecast/Makefile.am

@@ -4,7 +4,7 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir)/src/libcharon
 
 AM_CFLAGS = \
-	$(PLUGIN_CFLAGS)
+	$(PLUGIN_CFLAGS) $(libiptc_CFLAGS)
 
 if MONOLITHIC
 noinst_LTLIBRARIES = libstrongswan-forecast.la
@@ -18,4 +18,4 @@ libstrongswan_forecast_la_SOURCES = \
 	forecast_plugin.h forecast_plugin.c
 
 libstrongswan_forecast_la_LDFLAGS = -module -avoid-version
-libstrongswan_forecast_la_LIBADD  = -lip4tc
+libstrongswan_forecast_la_LIBADD = $(libiptc_LIBS)