Add a few missing scons flags

[dementive]

I'm looking into converting my module code over to godot-cpp and there are a lot of scons flags that I use in my build system for my module that currently aren't in godot-cpp. I added the one's that I care about here, these are:

• All of the "Environment flags" like ccflags, cxxflags, and others
• The linker flag on linux so I can use mold to link faster.
• The fast_unsafe flag because it makes incremental rebuilds noticeably faster.

I didn't write any of this code either. I just copied it straight from godot's build system. I'm not sure if the way I did it is ideal, like it feels kind of wrong to put get_compiler_version method in the linux file but there is no common methods.py file here like there is in the godot build system so I'm not sure where would be the best place to put that.

I'd also like to add all of the flags for the sanitizers: use_ubsan use_asan use_lsan use_tsan use_msan

In a future PR if possible.