Due to the previous SSL exception we coudln't restrict the collected
CFLAGS/LDFLAGS to those of enabled options, so all of them were
considered if set. The problem is that it would prevent simply
disabling a build option without unsetting its xxx_CFLAGS or _LDFLAGS
values if those had incompatible values (e.g. -lfoo).

Now that only existing options are listed in collect_opts_flags, we
can safely check that the option is set and only consider its settings
in this case. Thus OT_LDFLAGS will not be used if USE_OT is not set
for example.

By creating USE_SSL and enabling it when USE_OPENSSL is set, we can
get rid of the special case that was made with it regarding cflags
collect and when resetting options. The option doesn't need to be
manually set, though in the future it might prove useful if other
non-openssl API are supported.

It's getting complicated to configure includes and lib dirs for
OpenSSL API variants such as WolfSSL, because some settings are
common and others are specific but carry a prefix that doesn't
match the USE_* rule scheme.

This patch simplifies everything by considering that all SSL libs
will use SSL_INC, SSL_LIB, SSL_CFLAGS and SSL_LDFLAGS. That's much
more convenient. This works thanks to the settings collector which
explicitly checks the SSL_* settings. When USE_OPENSSL_WOLFSSL is
set, then USE_OPENSSL is implied, so that there's no need to
duplicate maintenance effort.

The new function collect_opts_flags now scans all USE_* options defined
in use_opts and appends the corresponding *_CFLAGS and *_LDFLAGS to
OPTIONS_{C,LD}FLAGS respectively. This will be useful to get rid of all
the individual concatenations to these variables.

A lot of _SRC, _INC, _LIB etc variables are set and expected to be
initialized to an empty string by default. However, an in-depth
review of all of them showed that WOLFSSL_{INC,LIB}, SSL_{INC,LIB},
LUA_{INC,LIB}, and maybe others were not always initialized and could
sometimes leak from the environment and as such cause strange build
issues when running from cascaded scripts that had exported them.

The approach taken here consists in iterating over all USE_* options
and unsetting any _SRC, _INC, _LIB, _CFLAGS and _LDFLAGS that follows
the same name. For the few variable names options that don't exactly
match the build option (SSL & WOLFSSL), these ones are specifically
added to the list. The few that were explicitly cleared in their own
sections were just removed since not needed anymore. Note that an
"undefine" command appeared in GNU make 3.82 but since we support
older ones we can only initialize the variables to an empty string
here. It's not a problem in practice.

We're now certain that these variables are empty wherever they are
used, and that it is possible to just append to them, or use them
as-is.

The features list that appears in -vv appears in a random order, which
always makes it a pain to look for certain features. Let's sort it.

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.