contrib: mingw64: fix libraries building in parallel

Using the same build folder in parallel results in issues. The CI works because it reverts to -j1. But it's considerably slower.

changed milestone to %4.0

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added MRStatus::Accepted label and removed MRStatus::Reviewable label

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added 37 commits

• 601b787e...5a30e9b9 - 36 commits from branch videolan:master
• 635962fd - contrib: mingw64: fix libraries building in parallel

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merged