build: add configure option to disable ipv4 iptables

This patch complements the previous one.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/configure.ac b/configure.ac
index eefcdac..152ad59 100644
--- a/configure.ac
+++ b/configure.ac
@@ -31,6 +31,9 @@
 	[Path where to install Xtables extensions [[LIBEXECDIR/xtables]]]),
 	[xtlibdir="$withval"],
 	[xtlibdir="${libexecdir}/xtables"])
+AC_ARG_ENABLE([ipv4],
+	AS_HELP_STRING([--disable-ipv4], [Do not build iptables]),
+	[enable_ipv4="$enableval"], [enable_ipv4="yes"])
 AC_ARG_ENABLE([ipv6],
 	AS_HELP_STRING([--disable-ipv6], [Do not build ip6tables]),
 	[enable_ipv6="$enableval"], [enable_ipv6="yes"])
@@ -54,6 +57,7 @@
 
 AM_CONDITIONAL([ENABLE_STATIC], [test "$enable_static" = "yes"])
 AM_CONDITIONAL([ENABLE_SHARED], [test "$enable_shared" = "yes"])
+AM_CONDITIONAL([ENABLE_IPV4], [test "$enable_ipv4" = "yes"])
 AM_CONDITIONAL([ENABLE_IPV6], [test "$enable_ipv6" = "yes"])
 AM_CONDITIONAL([ENABLE_DEVEL], [test "$enable_devel" = "yes"])
 AM_CONDITIONAL([ENABLE_LIBIPQ], [test "$enable_libipq" = "yes"])