Mk/bsd.options.mk: Skip option helpers for excluded options

[jrm]

Summary

Consider the following example. Some port option, FOO, does not make
sense with some flavor because the feature behind FOO should always be
enabled when the flavor is chosen. Before this change, when the port's
Makefile included, e.g., FOO_CONFIGURE_WITH=foo, even though FOO was
excluded, --without-foo would be added to CONFIGURE_ARGS. With this
change, do not process option helpers for such an option.

Test Plan

Will ask for a exp-run.

[jrm]

Adding some IRC discussion here.

<bapt> in your flavor the option should not even exist for a start
<bapt> so your should not need D44929

<jrm> bapt: If there are, say, 4 flavors, it makes more sense (to me) to exclude those options from that one flavor rather than adding them to the three other flavors.
<jrm> bapt: But [in any case], it seems better not to mess with option helpers for excluded options, no?

Let's say the option FOO does not make sense on some *arch*, so we exclude it. Should we still add things such as --without-foo to CONFIGURE_ARGS because the port contains FOO_CONFIGURE_WITH=foo?