cmake: Revert CMake's way to handle definitions

[hebasto]

This PR adjusts CMake scripts in such a way that makes removing #include <config/bitcoin-config.h> from sources is unnecessary.

Effectively, this makes Autotools and CMake builds comparable.

Addresses bitcoin#30454 (comment) and bitcoin#30454 (comment).

[hebasto]

Friendly ping @maflcko @theuni @fanquake @TheCharlatan :)

[theuni]

Are these going to remain as reverts on top of the original changes?

[hebasto]

Are these going to remain as reverts on top of the original changes?

No. I'm going to squash these commits with the past relevant commits during the next sync/rebase round.

[hebasto]

Rebased on top of the merged #268.

[hebasto]

Are these going to remain as reverts on top of the original changes?

No. I'm going to squash these commits with the past relevant commits during the next sync/rebase round.

The commit messages have been updated to reflect this intention.

[theuni]

I believe this is correct. Hard to tell without doing a full rebase, but the only related code-change in the diff from pre-cmake is src/qt/guiutil.cpp, which had its bitcoin-config.h include removed for the static plugin change.

If there are any mistakes here, I think they'll be obvious post-rebase.

utACK f440d33