BUILD: makefile: properly pass CC to sub-projects

The "poll" and "tcploop" sub-projects have their own makefiles. But
since the cmd_* commands were migrated from "echo" to $(info) with
make 3.81, the command is confusingly displayed in the top-level
makefile before entering the directory, even making one think that
the build occurred.

Let's instead propagate the verbosity level through the sub-projects
and let them adapt their own cmd_CC. For now this peans a little bit
of duplication for poll and tcploop.
diff --git a/Makefile b/Makefile
index 3e765ae..6902c37 100644
--- a/Makefile
+++ b/Makefile
@@ -1031,13 +1031,13 @@
 	$(cmd_LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)
 
 dev/poll/poll:
-	$(Q)$(MAKE) -C dev/poll poll CC='$(cmd_CC)' OPTIMIZE='$(COPTS)'
+	$(Q)$(MAKE) -C dev/poll poll CC='$(CC)' OPTIMIZE='$(COPTS)' V='$(V)'
 
 dev/qpack/decode: dev/qpack/decode.o
 	$(cmd_LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)
 
 dev/tcploop/tcploop:
-	$(Q)$(MAKE) -C dev/tcploop tcploop CC='$(cmd_CC)' OPTIMIZE='$(COPTS)'
+	$(Q)$(MAKE) -C dev/tcploop tcploop CC='$(CC)' OPTIMIZE='$(COPTS)' V='$(V)'
 
 dev/udp/udp-perturb: dev/udp/udp-perturb.o
 	$(cmd_LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)