Statically link SDL3 on Linux into self-contained ELF

- Add explicit elif defined(linux) branch that statically links SDL3
  (-Wl,-Bstatic -lSDL3 -Wl,-Bdynamic -pthread -lm) from a source-built
  libSDL3.a in /usr/local/lib.
- Document building latest stable SDL3 (3.4.20) from source with a
  static library, and update Build & Run / Dependencies docs for the
  three-branch (Windows/Linux/macOS) linking scheme.
