There's quite a large barely readable functions block in the makefile
dedicated to compiler option support. It provides no value here and
makes it harder to find user-configurable stuff, so let's move it to
include/make/compiler.mk to keep the makefile a bit cleaner. It's better
to keep the options themselves in the makefile however.
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.
