iptables: extensions/GNUMakefile.in use CPPFLAGS

"All other Makefiles add CPPFLAGS to ${COMPILE} (automake), but GNUmakefile.in
 doesn't set it."

http://bugs.debian.org/665286

Signed-off-by: Laurence J. Lane <ljlane@debian.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/extensions/GNUmakefile.in b/extensions/GNUmakefile.in
index 14e7c57..c5d8844 100644
--- a/extensions/GNUmakefile.in
+++ b/extensions/GNUmakefile.in
@@ -21,7 +21,7 @@
 kinclude_CPPFLAGS  = @kinclude_CPPFLAGS@
 
 AM_CFLAGS       = ${regular_CFLAGS}
-AM_CPPFLAGS     = ${regular_CPPFLAGS} -I${top_builddir}/include -I${top_builddir} -I${top_srcdir}/include ${kinclude_CPPFLAGS}
+AM_CPPFLAGS     = ${regular_CPPFLAGS} -I${top_builddir}/include -I${top_builddir} -I${top_srcdir}/include ${kinclude_CPPFLAGS} ${CPPFLAGS}
 AM_DEPFLAGS     = -Wp,-MMD,$(@D)/.$(@F).d,-MT,$@
 AM_LDFLAGS      = @noundef_LDFLAGS@