BUILD: makefile: properly pass CC to sub-projects

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.