[4.0] SCons: Fix BoolVariable import for SCons 4.8.0+

[akien-mga]

See godotengine/godot#94410 and https://stackoverflow.com/questions/78746956/error-name-boolvariable-is-not-defined-when-using-scons-on-godot-with-c for details.

Not needed in master as this was already changed by #1504.

Should be cherry-picked for 4.1 and 4.0, doesn't seem needed in 3.x/3.5.

[Faless]

See discussion in #1518 (which this PR fixes)

[dsnopek]

I intend to cherry-pick #1504, so a separate PR just for this change isn't necessary

UPDATE: Er, well, I wasn't planning to cherry-pick to 4.0, so if we want this in 4.0 that probably will need it's own PR. We've kept the build system for 4.1, 4.2 and master in pretty close sync, but 4.0 is quite different at this point.

[dsnopek]

Here's the next 4.2 cherry-picking PR (which includes this changes via PR #1504): #1527

[dsnopek]

And here's the 4.1 cherry-pick: #1529

So, at this point, maybe this PR could be reworked to bring this change to the 4.0 branch, since I don't intend to cherry-pick PR #1504 to 4.0?

[akien-mga]

So, at this point, maybe this PR could be reworked to bring this change to the 4.0 branch, since I don't intend to cherry-pick PR #1504 to 4.0?

Done!

[dsnopek]

Thanks!

[dsnopek]

I suspect the CI builds are stuck because we didn't cherry-pick PR #1496 to 4.0. I just added the cherrypick:4.0 label there

[dsnopek]

PR #1533 does the few cherry-picks we have for 4.0. Let's see if that manages to finish CI!

[dsnopek]

Rebasing this on the latest 4.0 branch will probably get the CI passing, now that PR #1533 is merged