build: add build option suppress_all_error_on_warn

[mhdawson]

Add an option to suppress cases where warnings are on by default

[nodejs-github-bot]

Review requested:

• @nodejs/gyp

[mhdawson]

Address comments in #56615 (comment) that there should be a way to opt out of all cases where Warnings are turned into errors.

I looked at the history to see if there was any discussion in Werror=unused-result was added
and I did not find anything specific to that option being added.

Given that the existing configuration option --error_on_warn would have already caught the unused-result if it was turned on, my assumption is that the desired behaviour was that -Werror=unused-result, be on by default and if you used the --error-on-warn configuration that would turn on/off additiona validation.

I agree we should make it possible to turn off all instances where warnings are turned into errors, and that is added in this PR.

The alternative would be to remove the -Werror=unused-result definition and have no enforcement by default but I don't think that meets the original intent. It is not possible to use the existing flag to get the existing behaviour and provide a way to disable all cases where warnings are turned into errors.

[mhdawson]

@khardix, @jackorp FYI

[bnoordhuis]

I think you or the reporter misunderstands what that flag does. An unused result warning is a bug. It's not harmless. Disabling the build error makes no sense at all. Fix the error instead.