Don't compile Bounce2 unless BUTTON_0 or BUTTON_1 are defined

[robertlipe]

Description

Like #788 and #789: don't download, install, compile Debounce unless it's used.

Contributing requirements

• [x] I read the contribution guidelines in CONTRIBUTING.md.
• [x] I understand the BlinkenPerBit metric, and maximized it in this PR.
• [x] I selected main as the target branch.
• [x] All code herein is subjected to the license terms in COPYING.txt.

[robertlipe]

I tried. The cases that are hard to pencil whip are the ones like the build failures in 789. The best regex skills still won't hit cases of inherited flags.

The safety net is that the includes are (now?) guarded by those same preprocessor tests. If there's not a libdeps, the packages won't get checked out and installed into every env (the motivator for these exercises) so the build fails with the library headers not being found. I thus think these are safe.

I could be wrong.

I also won't die of shock if these three independent PRs have merge conflicts. If they do, I'll fix them after Sunday.

[rbergen]

Alright, I'll take the risk on this one. I appreciate the honest and detailed explanation on how sure you are about this.