[SCons] Add warnings build options

[Faless]

Enable scons "warnings=extra" and "werror=yes" in CI.

Fix few spotted warnings.

Draft status:

• Unexplained duplicated-branches CI gcc error (cannot reproduce locally with gcc 10) when building test
• The changes in binder_common are slightly different from the upstream godot (include almost the whole file), and would like an opinion.

[Faless]

scons part of #999 , thanks a lot to @asmaloney for doing the initial work on fixing warnings.

[asmaloney]

Excellent! I'm CI-testing my cmake changes right now too , so I will wait for this to be merged.

[asmaloney]

This is odd because I'm not getting any warnings on the files you changed... I've checked my option conversion a couple of times and it looks OK (granted my ☕ level is low). I wonder if I'm missing something...

[Faless]

@asmaloney the error with the scons CI seems to come from building the test library, and the cmake counterpart seems to always use the isystem strategy after #1002 .

I wonder if we should do the same with the scons builds.