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.

configure.ac

@@ -1078,6 +1078,12 @@ if test x$xauth_pam = xtrue; then
 	AC_CHECK_HEADER([security/pam_appl.h],,[AC_MSG_ERROR([PAM header security/pam_appl.h not found!])])
 fi
 
+if test x$connmark = xtrue -o x$forecast = xtrue; then
+	PKG_CHECK_MODULES(libiptc, [libiptc])
+	AC_SUBST(libiptc_CFLAGS)
+	AC_SUBST(libiptc_LIBS)
+fi
+
 if test x$capabilities = xnative; then
 	AC_MSG_NOTICE([Usage of the native Linux capabilities interface is deprecated, use libcap instead])
 	# Linux requires the following for capset(), Android does not have it,

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)