I've improved the package build options section just to be more clear:

https://github.com/voidlinux/void-packages#package-build-options

- `build_options_default` is what makes the pkg build with those options by 
default, unless the
user disables them.
- use `./xbps-src -o opt,~opt2 <target> pkg` to build a pkg with your 
custom options.
- options can also be specified in `etc/conf`.

The `vopt` functions are helpers to conditionalize actions depending on the 
option being tested,
the manual documents this correctly, please refer to it for more 
information.