Fix make tags fail on non-default tag names

When ctags overrides default tags filename (e.g. `-f .tags`) `make tags`
is failed because it assumes to see default `tags` filename:

    sort: cannot read: tags: No such file or directory

This commit explicitly specifies "tags" filename for tags generation.

Author: dzeban

Makefile.pre.in

@@ -1555,7 +1555,7 @@ autoconf:
 tags::
 	cd $(srcdir); \
 	ctags -w Include/*.h; \
-	for i in $(SRCDIRS); do ctags -w -a $$i/*.[ch]; \
+	for i in $(SRCDIRS); do ctags -f tags -w -a $$i/*.[ch]; \
 	done; \
 	LC_ALL=C sort -o tags tags