Merge bitcoin/bitcoin#29494: build: Assume HAVE_CONFIG_H, Add IWYU pragma keep to bitcoin-config.h includes

fa09451f8e Add lint check for bitcoin-config.h include IWYU pragma (MarcoFalke)
dddd40ba82 scripted-diff: Add IWYU pragma keep to bitcoin-config.h includes (MarcoFalke)

Pull request description:

  The `bitcoin-config.h` includes have issues:

  * The header is incompatible with iwyu, because symbols may be defined or not defined. So the `IWYU pragma: keep` is needed to keep the include when a symbol is not defined on a platform. Compare the previous discussion in https://github.com/bitcoin/bitcoin/pull/29408#issuecomment-1948959711
  * Guarding the includes by `HAVE_CONFIG_H` is verbose and brittle. Now that all build config dependencies have been removed from low level headers, the benefits are questionable, and the guard can be removed. The linter could also be tricked by guarding the include by `#if defined(HAVE_C0NFIG_H)` (`O` replaced by `0`). Compare the previous discussion in https://github.com/bitcoin/bitcoin/pull/29404#discussion_r1483189853 .

ACKs for top commit:
  achow101:
    ACK fa09451f8e
  TheCharlatan:
    ACK fa09451f8e
  hebasto:
    re-ACK fa09451f8e, only rebased since my recent [review](https://github.com/bitcoin/bitcoin/pull/29494#pullrequestreview-2028864535) (`timedata.cpp` removed in https://github.com/bitcoin/bitcoin/pull/29623).

Tree-SHA512: 47cb973f7f24bc625acc4e78683371863675d186780236d55d886cf4130e05a78bb04f1d731aae7088313b8e963a9677cc77cf518187dbd99d776f6421ca9b52

src/init.cpp

@@ -3,9 +3,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
+#include <config/bitcoin-config.h> // IWYU pragma: keep
 
 #include <init.h>