- WITH_WINDOWS_EXTERNAL_MANIFEST is now required even for ninja
builds, this previously was not the case. so just apply it
across all build flavours on windows.
- fmsc-version=1922 is no longer needed, was required back when
support for clang on windows landed, is fine without it now.
- /MP gives better build performance when building from the IDE
makes no difference for a ninja build which wasn't struggling in
that department anyhow.
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