Document flags used to build with system dependencies

[PureTryOut]

Split off from #1868

Distributions don't want the software they ship to use internal libraries when they can be used system wide. It seems dotnet can be built using several system dependencies, but they seem to be undocumented at the moment. It would make sense to document those flags at https://github.com/dotnet/source-build/blob/release/3.1/Documentation/packaging-installation.md.

Think of flags like -p:UseSystemLibunwind=true.