The static Makefile is the only build. GitLab CI and the GitHub workflows
build with it, with the same lanes and toolchains; the MSVC workflow and
the CMake-only macOS universal-binary script go, as nothing builds them.
The Readme describes the Makefile build.
* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
individual Externals
* corrects some cruft that accumulated and probably contributed
to msbuild overbuilding
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