SCons: Make lto=auto prefer ThinLTO over full LTO for LLVM targets

This speeds up build time considerably for these platforms compared to
using `lto=full`, which is sadly single-threaded with LLVM, unlike GCC.

Changes to default behavior of `lto=auto` (i.e. `production=yes`):
- Linux: Prefer ThinLTO for LLVM
- Web: Prefer ThinLTO
- Windows: Prefer ThinLTO for llvm-mingw

The following LLVM targets don't use LTO by default currently, which
needs to be assessed further (gains from LLVM LTO on performance need
to be weighed against the potential size increase from heavy inlining):
- Android
- iOS
- macOS
- Windows clang-cl

platform/windows/detect.py

@@ -762,8 +762,8 @@ def configure_mingw(env: "SConsEnvironment"):
 
     ## LTO
 
-    if env["lto"] == "auto":  # Full LTO for production with MinGW.
-        env["lto"] = "full"
+    if env["lto"] == "auto":  # Enable LTO for production with MinGW.
+        env["lto"] = "thin" if env["use_llvm"] else "full"
 
     if env["lto"] != "none":
         if env["lto"] == "thin":