Remove the _BuildingMidl item group and evaluate ExcludedFromBuild inline.

[jlaanstra]

Fixes #1050

[kennykerr]

Thanks!

[sylveon]

I added that to avoid duplicating the evaluation, but it can't be MSBuild without surprises

[kennykerr]

@sylveon It was causing problems with some internal builds. 🤷‍♂️

[jlaanstra]

I added that to avoid duplicating the evaluation, but it can't be MSBuild without surprises

The problem is that once the target has run, the _BuildingMidl item group can easily become stale if a later target adds items to the Midl item group as those won't be picked up by _BuildingMidl. That's exactly what happened in the internal project.

[jlaanstra]

@sylveon if you have time could you make sure this change still works for your use case?

[sylveon]

Yep, still works for me!

[jlaanstra]

@kennykerr let's get this merged!