Dissolve msh3_headers, cleanup

[dg0yt]

This is probably the most intrusive PR in today's series.

• Revise QUIC_... variables and either remove or migrate to MSH3.
  • The vendored msquic build overrides the variables.
  • Let /Qspectre potentially take effect. (Note that HAS_SPECTRE is not wired to a visible option or active check in msh3. Maybe it should be removed entirely. The user can pass this with toolchain or variables.)
  • Drop _GNU_SOURCE. (It was not needed in the past.)
• Remove the msh3_header interface target:
  • The actual include dir is a public property of msh3 (subject to INSTALL_INTERFACE/BUILD_INTERFACE).
  • C++17 is enough to build the lib now.
    Building the lib needs at least C++20 for "use of designated initializers" (MSVC).
    (Not exported as a transitive usage requirement.)
  • WIN32_LEAN_AND_MEAN is a project-wide definition. (Not exported as a transitive usage requirement.)
  • VER_BUILD_ID and VER_SUFFIX are used only by msh3. (Not exported as a transitive usage requirement.) I didn't notice problems on non-windows.
• Add additional includes to msh3_app. Not sure why the Ubuntu build didn't complain before, but it did now.

Linking msh3app and mhs3test now looks as trivial as it should, with proper use of CMake.