Implement use_static_cpp flag for Linux

[unvermuthet]

Without statically linking libstdc++ you run into backward-compatibility issues when trying to run a GDExtension linked against a newer version of libstdc++, on a machine that runs an older version. This will also let us upgrade CI runner images to the latest version on godot-cpp-template.

Godot's buildsystem already does this. https://github.com/godotengine/godot/blob/fc827bbe256e47e1d7ec6945deb9c1e79724dac9/platform/linuxbsd/detect.py#L499-L506

I'm fumbling in the dark with CMake. Open to letting someone else handle that part.

[unvermuthet]

As mentioned in the meeting, this PR will add another use_static_cpp flag. Two will now show up when running scons --help. One with the description for behavior on Windows and one describing the behavior on Linux (new). This is already happening with use_llvm. Adding these options conditionally like Godot's build system does, would require larger changes. For this PR I'd consider adding a note to the new flag description.

[unvermuthet]

Perhaps also adding - only effective when targeting Linux here makes sense?

[dsnopek]

I'm not sure this is worth noting. It's weird that we get the option twice, but I don't think having an extra note in the option makes that any clearer.

Also, I think this is basically a GCC option, and doesn't really have anything to do with Linux. If we were to try to consolidate the options (which I'm not necessarily saying we should), it would probably be to add this option when the compiler is GCC

Honestly, I think I'm personally fine merging this without solving the duplicate option situation. I think we could tackle that in a follow-up

[unvermuthet]

I was considering only adding the option once and consolidating the messaging. But there is a difference in behavior. When targeting Windows, it also adds -static in addition to the other two. The help text there is "Link MinGW/MSVC C++ runtime libraries statically".

I agree that it's okay to have them twice.

[enetheru]

given that, then have no problems with the changes, just trying to figure out how to approve them now.

[unvermuthet]

@enetheru Could you take a look at the CMake part, please?

[enetheru]

Even though there is duplication of the option, the changes are good.
A follow up consoidating the option and generator expression would be nice.

[dsnopek]

Thanks!

[unvermuthet]

This should be considered for the next round of godot-cpp 4.4 cherry picks.