Fix compiledb SCons tool availability

[adamscott]

Currently, scons compiledb compiledb=no fails to run, as the compiledb alias isn't set if the compiledb variable isn't True.

X@X godot $ scons -Q compiledb
Automatically detected platform: linuxbsd
Auto-detected 8 CPU cores available for build parallelism. Using 7 cores by default. You can override it with the -j argument.
Note: Using `execinfo=yes` for the crash handler as required on platforms where glibc is missing.
Building for platform "linuxbsd", architecture "x86_64", target "editor".
[Initial build] scons: *** Do not know how to make File target `compiledb' (/home/X/srg/godot/compiledb).  Stop.
[Time elapsed: 00:00:03.429]

This PR makes the compiledb tool available for being summoned directly.

The variable is only used to indicate that we want to run the compiledb tool on the main build.

[akien-mga]

I assume black isn't happy with keeping this included just before it's used?

[adamscott]

No, it's just that I thought that it could be a nice global variable. I can set it as it was, if you want.

[akien-mga]

Yeah I prefer to keep it close to where it's used. It's a hack, using a private API, so it's not something we should encourage to rely on.

We could even drop it and increase the min SCons version to 4.0.0, but that would remove support for Ubuntu 20.04 LTS, which is still commonly used to make custom builds.

[adamscott]

Couldn't we build install scons using pip on Ubuntu 20.04 LTS?

[adamscott]

By the way, I reverted back these lines.

[akien-mga]

Couldn't we build install scons using pip on Ubuntu 20.04 LTS?

We don't build on Ubuntu 20.04 LTS, but many users do. And yes they could install SCons with pip, but this adds some friction, and is not a suitable option everywhere (e.g. for distro packaging, though I doubt Ubuntu will start packaging Godot for Ubuntu 20.04).

[Riteo]

Looks fine!

[adamscott]

@akien-mga @Riteo Fixed up the superfluous // SCons. But I added/reordered comments to make it clearer where features begin and end.

[akien-mga]

Thanks!