Build tweaks related to tags files.

* lib-src/Makefile.in (tagsfiles): New variable.
(TAGS): Also depend on the source files.  Use our own etags program.
* lisp/Makefile.in (ETAGS): Add EXEEXT.
(lisptagsfiles1, lisptagsfiles2, lisptagsfiles3, lisptagsfiles4):
Remove.
(tagsfiles): New, replacing lisptagsfiles1 etc.
Remove irrelevant source files here rather than in the TAGS rule.
(${ETAGS}): New rule.
(TAGS): Also depend on the etags executable.
* lwlib/Makefile.in (EXEEXT): New, set by configure.
(ETAGS): Add EXEEXT.
(${ETAGS}): New rule.
(ctagsfiles): Use "wildcard".
(TAGS): Also depend on the etags executable.
* nt/Makefile.in (ETAGS, tagsfiles): New variables.
(${ETAGS}): New rule.
(TAGS): Fix dependencies.
* oldXMenu/Makefile.in (EXEEXT): New, set by configure.
(ETAGS): New variable, replacing $TAGS.  Use our own etags program.
Remove "-t" argument.
(${ETAGS}): New rule.
(tagsfiles): New variable.
(TAGS): New rule, with proper dependencies.
* src/Makefile.in (ETAGS): Add EXEEXT.  Add a build rule.
(ctagsfiles1, ctagsfiles2): Use "wildcard".
(ctagsfiles3): Remove.
(TAGS): Depend on etags.
(../lisp/TAGS, $(lwlibdir)/TAGS): Let the rules in the relevant
directories decide if updates are needed.

lwlib/Makefile.in

@@ -44,6 +44,7 @@ RANLIB=@RANLIB@
 AR = @AR@
 ARFLAGS = @ARFLAGS@
 MKDIR_P = @MKDIR_P@
+EXEEXT = @EXEEXT@
 
 LUCID_OBJS = lwlib-Xlw.o xlwmenu.o lwlib-Xaw.o
 MOTIF_OBJS = lwlib-Xm.o
@@ -128,15 +129,17 @@ distclean: clean
 bootstrap-clean maintainer-clean: distclean
 	rm -f TAGS
 
+ETAGS = ../lib-src/etags${EXEEXT}
 
-ETAGS = ../lib-src/etags
+${ETAGS}: FORCE
+	${MAKE} -C ../lib-src $(notdir $@)
 
-ctagsfiles= $(srcdir)/*.[ch]
+ctagsfiles= $(wildcard ${srcdir}/*.[ch])
 
-TAGS: $(ctagsfiles)
-	"$(ETAGS)" $(ctagsfiles)
+FORCE:
+.PHONY: tags FORCE
 tags: TAGS
-.PHONY: tags
-
+TAGS: ${ETAGS} $(ctagsfiles)
+	${ETAGS} $(ctagsfiles)
 
 ### Makefile.in ends here