Make sure not to generate the compile_commands.json when not asked

[hakro]

Did not create an issue for this, as the fix is pretty simple.

Scons seems to always generate the compile_commands.json, regardless of whether the flag compiledb is set or not, and even with an explicit compiledb=no.

There are a whopping 3-4 seconds out of 6-10 seconds of build time saved on my machine when I don't need to recreate that file, which is pretty nice when iterating fast on small changes.

[akien-mga]

I confirm the code was similar in 4.2, so this is a regression fix:

https://github.com/godotengine/godot/blob/3504c98c1233bbd2506e89ce46509bc79afaec17/SConstruct#L952L963

[akien-mga]

Thanks!