Make 'make depend' fail as soon as a sub-command it invokes fails.

With this commit, 'make alldepend' will stop and report an error as
soon as one of the invoked command fails, which was not the case before.

This commit thus makes it possible to test that 'make alldepend' works
in an automated way.

Makefile

@@ -973,7 +973,7 @@ alldepend:
 else
 alldepend: depend
 	for dir in $(subdirs); do \
-	  $(MAKE) -C $$dir depend; \
+	  $(MAKE) -C $$dir depend || exit; \
 	done
 endif
 
@@ -1277,7 +1277,7 @@ partialclean::
 depend: beforedepend
 	(for d in utils parsing typing bytecomp asmcomp middle_end \
 	 middle_end/base_types driver toplevel; \
-	 do $(CAMLDEP) -slash $(DEPFLAGS) $$d/*.mli $$d/*.ml; \
+	 do $(CAMLDEP) -slash $(DEPFLAGS) $$d/*.mli $$d/*.ml || exit; \
 	 done) > .depend
 	$(CAMLDEP) -slash $(DEPFLAGS) -native \
 		-impl driver/compdynlink.mlopt >> .depend