Build adjustments

A few build system changes.
	* ip6tables needs IP6T_LIB_DIR
	* correctly trigger rebuild of master manpages when
	  submanpages have been touched

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de.
diff --git a/configure.ac b/configure.ac
index e616329..b90c4a7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -40,7 +40,7 @@
 	-D_REENTRANT -Wall -Waggregate-return -Wmissing-declarations \
 	-Wmissing-prototypes -Wredundant-decls -Wshadow -Wstrict-prototypes \
 	-Winline -pipe -DIPTABLES_VERSION=\\\"$PACKAGE_VERSION\\\" \
-	-DIPT_LIB_DIR=\\\"\${iptdir}\\\"";
+	-DIPT_LIB_DIR=\\\"\${iptdir}\\\" -DIP6T_LIB_DIR=\\\"\${iptdir}\\\"";
 kinclude_CFLAGS="-I\"$kbuilddir/include\" -I\"$ksourcedir/include\"";
 
 # Remove workarounds soon