build: restructure Makefile for include/ directory

This patch will support adding libiptc to the headers list in future.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/Makefile.am b/Makefile.am
index dbdfa97..be3f55e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -5,6 +5,9 @@
 
 AM_CFLAGS        = ${regular_CFLAGS} -I${top_builddir}/include -I${top_srcdir}/include ${kinclude_CFLAGS}
 SUBDIRS          = extensions
+if ENABLE_DEVEL
+SUBDIRS         += include
+endif
 if ENABLE_LIBIPQ
 SUBDIRS         += libipq
 endif
@@ -78,9 +81,6 @@
                    iptables-xml.8 ip6tables.8 ip6tables-restore.8 \
                    ip6tables-save.8
 CLEANFILES       = iptables.8 ip6tables.8
-if ENABLE_DEVEL
-include_HEADERS  = include/xtables.h
-endif
 
 if ENABLE_STATIC
 sbin_PROGRAMS += iptables-static ip6tables-static
diff --git a/configure.ac b/configure.ac
index 888ba42..3277666 100644
--- a/configure.ac
+++ b/configure.ac
@@ -78,6 +78,7 @@
 libxtables_vmajor=$(($libxtables_vcurrent - $libxtables_vage));
 AC_SUBST([libxtables_vmajor])
 
-AC_CONFIG_FILES([Makefile extensions/GNUmakefile libipq/Makefile
+AC_CONFIG_FILES([Makefile extensions/GNUmakefile include/Makefile
+	libipq/Makefile
 	include/xtables.h include/iptables/internal.h xtables.pc])
 AC_OUTPUT
diff --git a/include/Makefile.am b/include/Makefile.am
new file mode 100644
index 0000000..6206bd2
--- /dev/null
+++ b/include/Makefile.am
@@ -0,0 +1,8 @@
+# -*- Makefile -*-
+
+include_HEADERS =
+nobase_include_HEADERS = xtables.h
+
+if ENABLE_LIBIPQ
+include_HEADERS += libipq/libipq.h
+endif
diff --git a/libipq/Makefile.am b/libipq/Makefile.am
index d4245e7..f19950f 100644
--- a/libipq/Makefile.am
+++ b/libipq/Makefile.am
@@ -4,7 +4,6 @@
 
 libipq_a_SOURCES = libipq.c
 lib_LIBRARIES    = libipq.a
-include_HEADERS  = ${top_srcdir}/include/libipq/libipq.h
 man_MANS         = ipq_create_handle.3 ipq_destroy_handle.3 ipq_errstr.3 \
                    ipq_get_msgerr.3 ipq_get_packet.3 ipq_message_type.3 \
                    ipq_perror.3 ipq_read.3 ipq_set_mode.3 ipq_set_verdict.3 \