Device Atlas' dummy lib will use a C++ file when built with cache
support, so for completeness we'll have to pretty-print it as well.
Let's define cmd_CXX.
It's better to see "make" entering a subdir than seeing nothing, so
let's use a command name for make. Since make 3.81, "+" needs to be
prepended in front of the command to pass the job server to the subdir.
The $(Q), $(V), $(cmd_xx) handling needs to be reused in sub-project
makefiles and it's a pain to maintain inside the main makefile. Let's
just move that into a new subdir include/make/ with a dedicated file
"verbose.mk". It slightly cleans up the makefile in addition.
