The TAGS rule in src/Makefile.in can fail when etags is missing

src/Makefile.in

@@ -230,14 +230,13 @@ selfbuild:
 
 # Make sure to also include *.d files; and match DPP's idiosyncracies
 # like "@si::foo" and "@(defun si::foo".
+# This rule is allowed to fail when etags does not exist.
 TAGS:
-	if which etags ; then \
-	srcfiles=`find $(srcdir)/c/ $(srcdir)/h/ -name '*.[chd]'`; \
+	echo > $@;
+	-srcfiles=`find $(srcdir)/c/ $(srcdir)/h/ -name '*.[chd]'`; \
 	etags --language=c    -o $@ $$srcfiles && \
 	etags --language=none -o $@ --append \
 	      --regex='/@\([-:*a-zA-z]+\)/\1/' \
 	      --regex='/@(defun \([-:*a-zA-z]+\)/\1/' \
-	      $$srcfiles; \
-	else \
-	echo > $@; \
-	fi
+	      $$srcfiles
+