build: optional MIR2X_CLIENT_ONLY and Apple Clang flags

[iamcheyan]

Lets the project be configured and built on macOS without manual
CMake hacking. Linux GCC builds are unchanged.

Source-level companion PRs (independent, but you'll want them
merged together for the macOS build to actually compile end-to-
end): #57 (sol2 v3 rename), #58 (explicit headers / strict
conformance).

Top-level CMakeLists.txt

• New MIR2X_CLIENT_ONLY option (default OFF — preserves
  current behaviour). When ON, server/ and tools/ are skipped.
  This is useful on macOS, where the server still needs brew LLVM
  to build (Apple Clang 17 hits an internal compiler crash on
  server/src/player.cpp's nested-lambda pattern in
  clang::Expr::EvaluateAsInitializer — reproducible; happy to
  file upstream at LLVM if useful) and the tools need extra
  FLTK 1.4 / Cairo wiring beyond the scope of this PR.
• For Apple Clang only: don't pass -fcoroutines (GCC-only flag;
  Clang has C++20 coroutines on by default) and don't promote
  warnings to errors with -Werror (Clang's stricter warnings
  trip on otherwise-fine code; once everything's clean we can
  switch it back on).

client/src/CMakeLists.txt

• On APPLE, append -L/opt/homebrew/lib so the linker finds
  SDL2_ttf / SDL2_gfx / SDL2_image / SDL2_mixer installed
  via Homebrew on Apple Silicon.

common/src/CMakeLists.txt

• Link stdc++fs only for GCC. libc++ (Apple Clang, brew LLVM)
  has std::filesystem in the main runtime; linking stdc++fs
  there fails or is a no-op.
• Explicitly add SDL2 / FLTK header dirs to the common target's
  include paths so subtargets that pull in common headers don't
  need to re-resolve them.

Tested

• Linux: not retested in this PR; changes are gated behind
  APPLE / Clang / MIR2X_CLIENT_ONLY so the existing GCC
  path is untouched. Happy to verify on a Linux box if you'd
  like before merging.
• macOS arm64, Apple Clang 17: cmake -DMIR2X_CLIENT_ONLY=ON ..
  configures and builds the client cleanly.
• macOS arm64, brew LLVM 22: full client+server build with
  MIR2X_CLIENT_ONLY=OFF works (alongside compat: sol::nil -> sol::lua_nil for sol2 v3 #57 + fix: add explicit STL headers and tighten two non-portable usages #58).