options/makeman: Handle __REQUIRED_OPTIONS

[cperciva]

Summary

Teach the code which generates src.conf.5 about __REQUIRED_OPTIONS;
without this change it gets confused and thinks that every option
turns the required options on.

[sjg]

tools/build/options/makeman

127

it might be safer to to use -V '${__REQUIRED_OPTIONS:ts,}' to get requireds and then case ",${requireds}," in *,${opt},*)

[cperciva]

Update per suggestion from sjg

[cperciva]

tools/build/options/makeman

127

Thanks! I was trying to figure out how to do that, but I didn't realize make supported substitutions in -V arguments.