share/mk: basic support for group options

[brooks]

Summary

Support group options where 1 of n values will be selected (or a default
value will be used). After processing an OPT_FOO will be set to one
value from FOO_OPTIONS for each FOO in SINGLE_OPTIONS. If the user
sets FOO that value will be used, otherwise FOO_DEFAULT will be used
(if FOO_DEFAULT is not explicitly defined it will be set to the first
value in FOO_OPTIONS).

This is somewhat inspired by OPTIONS_SINGLE from ports, but the
structure is quite different with a per-option variable a la MK_FOO.

[brooks]

Fix typo

[brooks]

Ports also has RADIO, GROUP, and MULTI for different set types, but this is the one I want now.

We might eventually want to interact with __DEFAULT_DEPENDENT_OPTIONS options, but I don't want to try to work that out without a use case or two.

[brooks]

@jrtc27 rightly points out I should add makeman (and thus showconfig) support. I'll look into that in followup commits.

[brooks]

• Add BROKEN_SINGLE_OPTIONS
• Add copy to kern.opts.mk
• Unset __SINGLE_OPTIONS

[imp]

I like this... And the make foo isn't too bad...

[brooks]

I did realize as I was working on makeman support that we should also implement what ports calls MULTI options to tame the LLVM backend support, but I'd like to land this stack first as I need to get back to the work that motivated it.

[emaste]

were -> where in the commit message

[emaste]

share/mk/bsd.mkopt.mk

34–37

I find the description a little confusing. If an example can reasonably be included I think it might make it more clear.

[brooks]

I've been debating if __FOO_DEFAULT and __FOO_OPTIONS should have the __ prefix. I think it's arguable that they are part of the public interface, but I'm not sure.

share/mk/bsd.mkopt.mk

34–37

I'm somewhat tempted to drop the FOO_DEFAULT:=${FOO_OPTIONS:[1]} thing and just make it an error to not set __FOO_DEFAULT. If it did that would the text be clearer as:

# For each option in __SINGLE_OPTIONS, OPT_FOO is set to FOO if
# defined and __FOO_DEFAULT if not.  Valid values for FOO are specified
# by __FOO_OPTIONS.

It's hard to work an example here since use to OPT_FOO is elsewhere.

[brooks]

Tweak comments

[emaste]

share/mk/bsd.mkopt.mk

34–37

I think requiring FOO_DEFAULT is fine

[brooks]

Require explicit __FOO_DEFAULT

[imp]

In D41659#949936, @brooks wrote:

Require explicit __FOO_DEFAULT

I'm agnostic on this being required. I'm good with it though