[libvpx] Add --disable-multithread option for libvpx

[Joy1024]

Add --disable-multithread option for libvpx

[Joy1024]

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[BillyONeal]

Can you explain what this does and why this is a feature? See https://learn.microsoft.com/vcpkg/contributing/maintainer-guide#features

In particular, it seems likely that someone might need to assert that this flag is not used, which means it can't be a feature.