build: separate AC variable replacements from xtables.h

It was/is a bit annoying that modifying xtables.h.in causes configure
to rerun. Split the @foo@ things into a separate file to bypass this.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
diff --git a/configure.ac b/configure.ac
index 861f5b3..4060f90 100644
--- a/configure.ac
+++ b/configure.ac
@@ -127,5 +127,5 @@
 	libiptc/Makefile libiptc/libiptc.pc
 	libiptc/libip4tc.pc libiptc/libip6tc.pc
 	libxtables/Makefile utils/Makefile
-	include/xtables.h include/iptables/internal.h])
+	include/xtables-version.h include/iptables/internal.h])
 AC_OUTPUT