SCons: Warn when passing unknown variables

[Repiteo]

Lifted largely from godot-cpp's implementation¹, albeit with formatting tweaked to be more in-line with recent SConstruct adjustments.

Example output:

Executing task: scons dev_build=yes compiledb=yes optimize=debug verbose=no tests=yes warnings=extra d3d12=yes module_text_server_fb_enabled=yes cpp_standard=23 ninja=no 

scons: Reading SConscript files ...
Automatically detected platform: windows
WARNING: Unknown SCons variables were passed and will be ignored:
        cpp_standard = 23
Auto-detected 32 CPU cores available for build parallelism. Using 31 cores by default. You can override it with the -j argument.

Footnotes

1. https://github.com/godotengine/godot-cpp/blob/e23b117ac32fdbeb0920f234e193e6d31307c0ad/SConstruct#L41-L46 ↩

[Repiteo]

Double-checked to make sure the regressions from #56551 are no longer applicable. The mono-stuff is no longer relevant as that logic appears to have been removed entirely, or otherwise relocated to not use the argument system. The only regression I could replicate was the profile option, and that came down to it simply… Not being an option. By adding that as a proper variable, everything works as expected!