xbps-src: new vopt_feature helper

[cinerea0]

Testing the changes

• I tested the changes in this PR: briefly

I didn't check this against every template where the pattern appears, but it works in the test cases I tried.

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This helper function reduces boilerplate for Meson's "feature" build options. Right now these are handled using the pattern -D<property>=$(vopt_if <option> enabled disabled), which occurs 78 times in our templates.

Note that this does not allow you to set the property to "auto", but I can't think of any situation where that would be desirable.

[Chocimier]

I would merge it but not under that name, even meson don't call that feat.
Also please join commits.

[eli-schwartz]

I would merge it but not under that name, even meson don't call that feat.

Or,

feat
noun

• an achievement that requires great courage, skill, or strength.

[cinerea0]

I would merge it but not under that name, even meson don't call that feat.

Since Meson refers to them as "features", I've renamed it to vopt_feature.

Also please join commits.

Done.

[eli-schwartz]

Prior art: https://devmanual.gentoo.org/eclass-reference/meson.eclass/index.html#lbAF

meson_use is equivalent to vopt_bool

meson_feature is equivalent to the proposed vopt_feature

They, too, do not support defining it to auto. A Gentoo USE flag is either enabled or disabled, and besides they explicitly want to enforce that automagic dependencies do not happen, since they don't build in chroots/containers. :D

[Chocimier]

Awaiting PR switching to new function.

[cinerea0]

Awaiting PR switching to new function.

I'll try to handle this tonight.