Fix build issues when there's no iptables present.

Patch from Matt Whitlock
See https://bugs.gentoo.org/show_bug.cgi?id=577464
Status: in theory submitted upstream by Lars Wendler.

--- iproute2-4.5.0/configure~	2016-03-14 23:02:31.000000000 +0000
+++ iproute2-4.5.0/configure	2016-03-17 13:24:17.634743197 +0000
@@ -169,10 +169,25 @@
 
 check_ipt()
 {
-	if ! grep TC_CONFIG_XT Config > /dev/null
+	if grep -q TC_CONFIG_XT Config
 	then
+		return
+	fi
+
+	cat >$TMPDIR/ipttest.c <<EOF
+#include <iptables.h>
+int main() { return 0; }
+EOF
+
+	if $CC -std=c90 -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL \
+		$(${PKG_CONFIG} libiptc --cflags --libs 2>/dev/null) -ldl >/dev/null 2>&1
+	then
+		echo "TC_CONFIG_IPT:=y" >>Config
 		echo "using iptables"
+	else
+		echo "no"
 	fi
+	rm -f $TMPDIR/ipttest.c $TMPDIR/ipttest
 }
 
 check_ipt_lib_dir()
--- iproute2-4.5.0/tc/Makefile~	2016-03-14 23:02:31.000000000 +0000
+++ iproute2-4.5.0/tc/Makefile	2016-03-17 13:18:18.686689985 +0000
@@ -88,7 +88,9 @@
 	CFLAGS += -DTC_CONFIG_XT_H
 	TCSO += m_xt_old.so
     else
-      TCMODULES += m_ipt.o
+      ifeq ($(TC_CONFIG_IPT),y)
+        TCMODULES += m_ipt.o
+      endif
     endif
   endif
 endif