Some macros and functions are barely understandable and are only used
to iterate over known options from the use_opts list. Better assign
them a name and move them into a dedicated file to clean the makefile
a little bit. Now at least "use_opts" only appears once, where it is
defined. This also allowed to completely remove the BUILD_FEATURES
macro that caused some confusion until previous commit.
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.
