BUILD: makefile: only consider settings from enabled options

BUILD: makefile: only consider settings from enabled options

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.