BUILD: makefile: allow to use CFLAGS to append build options

BUILD: makefile: allow to use CFLAGS to append build options

CFLAGS has always been a troublemaker because the variable was preset
based on other options, including dynamically detected ones, so
overriding it would just lose the original contents, forcing users
to resort to various alternatives such as DEFINE, ADDINC or SMALL_OPTS.

Now that the variable's usage was cleared, let's just preset it to
empty (and it MUST absolutely remain like this) and append it at the
end of the compiler's options. This will now allow to change an
optimization level, force a CPU type or disable a warning as users
commonly expect from CFLAGS passed to a makefile, and not to override
*all* the compiler flags as it has progressively become.