Add lto scons option

[Ivorforce]

The behavior mimics almost the exact behavior (minus "progress" options) of Godot scons:

• SConstruct (option declaration).
• Web
• Windows
• Linux
• macOS
• Android
• iOS

This is an active change (edit: not anymore, defaults to 'none' by default now): Once merged, all projects depending on godot-cpp (and updating the submodule) will gain LTO for release builds. I tested a similar setup in my own project, not the exact same though (because I don't have all combinations of windows, msvc, mingw and use_llvm available). However, since that it mimics what Godot does already, it should be good, given I did not make mistakes.

One important difference is that in my implementation (edit: not anymore, consistently 'none' by default now), "auto" is consistent across platforms ("full" in production, "none" in dev). I saw small, but consistent gains in all platforms, and don't see why LTO should be avoided in some platforms by default.

[Ivorforce]

Not sure why the windows builds are failing. I don't have this problem with any of my git runners. If this is related to the PR, we can default to no LTO on windows. Let me know what you think.

[Faless]

Looking good overall 🚀 (haven't tested a build yet).

Some changes are needed to match the default values with upstream godot.

[dsnopek]

Code-wise, this seems to be getting there!

However:

1. It'd be great if @Faless could give it one last review, and
2. This will need to be squashed into a single commit before it can be merged - see https://docs.godotengine.org/en/latest/contributing/workflow/pr_workflow.html#the-interactive-rebase

[Faless]

The code looks correct, but the commit message seems wrong?

The message states:

Add lto scons option, defaulting to "auto" ("full" in production, "none" in dev builds).

But there is no "production" option, and the default is none.

[Ivorforce]

Right, i forgot to change the final commit message after the changes. Should be good now!

[Faless]

LGTM! Nice work!

[dsnopek]

Thanks!