Makefile: add scripts/autoconf.mk, but do not use it yet

Add scripts/autoconf.mk in Makefile similar to scripts/autoconf.h
(i.e. use it in rules, e.g. remove it in clean rule, ...) but do
not use it for the build process (i.e. include it into Makefile) yet.

Makefile

@@ -178,7 +178,8 @@ distclean: clean
 		if [ -f $$i/Makefile ]; then $(MAKE) -i -C $$i distclean; fi; \
 	done;
 	-rm -f *~ .config .config.old scripts/autoconf.h .menuconfig \
-		Makefile.tmp .menuconfig.log scripts/defconfig.old
+		Makefile.tmp .menuconfig.log scripts/defconfig.old \
+		scripts/autoconf.mk \
 	find . -name '.#*' -exec rm -f {} \;
 
 scripts/lxdialog/lxdialog:
@@ -187,6 +188,9 @@ scripts/lxdialog/lxdialog:
 scripts/autoconf.h: .config
 	perl scripts/mk_autoconf.pl
 
+scripts/autoconf.mk: .config
+	perl scripts/mk_autoconf.pl
+
 cfgerror:
 	@echo ""
 	@echo "WARNING! Configure in $(ERRDIR) failed, disabling package"
@@ -199,7 +203,7 @@ cfgerror:
 #  - if a Makefile.in exists, make -f Makefile.in config
 #  - if a Makefile already exists, make config
 #
-subconfig: scripts/autoconf.h
+subconfig: scripts/autoconf.h scripts/autoconf.mk
 	@echo Selected subdirs: $(BUILD_ONLY) $(SUBDIRS)
 	@set -e; for i in `echo $(BUILD_ONLY) $(SUBDIRS)`; do \
 		if [ $$i = eicon ] ; then \