SCons: Add production=yes option to use production defaults

This is meant for users making custom builds to match the options used on
optimized, official builds.

This enables, on the platforms which support them:
- `use_static_cpp=yes` (portable binaries for Linux and Windows)
- `use_lto=yes` (link time optimizations - note: requires a lot of RAM!)
- `debug_symbols=no` (no debug symbols, smaller binaries)

Also abort when using MSVC with `production=yes`, as:
- It cannot optimize the GDScript VM like GCC or Clang do, leading to
  significant performance drops.
- Its LTO support is unreliable, at least used to trigger crashes last
  we tried it extensively.

All options can still be overridden if specified, and the `dev=yes` option
was changed to also support overrides.

platform/linuxbsd/detect.py

@@ -72,7 +72,7 @@ def get_opts():
         BoolVariable("use_tsan", "Use LLVM/GCC compiler thread sanitizer (TSAN))", False),
         BoolVariable("pulseaudio", "Detect and use PulseAudio", True),
         BoolVariable("udev", "Use udev for gamepad connection callbacks", True),
-        EnumVariable("debug_symbols", "Add debugging symbols to release/release_debug builds", "yes", ("yes", "no")),
+        BoolVariable("debug_symbols", "Add debugging symbols to release/release_debug builds", True),
         BoolVariable("separate_debug_symbols", "Create a separate file containing debugging symbols", False),
         BoolVariable("touch", "Enable touch events", True),
         BoolVariable("execinfo", "Use libexecinfo on systems where glibc is not available", False),