cmake: Rebase and backport C++20 changes

[hebasto]

The only goal of this PR is to backport changes from bitcoin#28349.

As they include changes to the depends build system, I decided to combine C++20 changes in CMake stuff with the rebasing of the cmake-staging branch on top of the master @ 3e691258d8789a4a89cce42e7e71b130491594d7 (the fixup/autosquash approach doesn't work well as it introduces a number of conflicts).

For me, the git diff cmake-staging..231208-linear -- CMakeLists.txt gives:

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -31,11 +31,14 @@ set(COPYRIGHT_HOLDERS "The %s developers")
 set(COPYRIGHT_HOLDERS_FINAL "The ${PROJECT_NAME} developers")
 set(PACKAGE_BUGREPORT "https://github.com/bitcoin/bitcoin/issues")
 
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/module)
 
 # Configurable options.
 # When adding a new option, end the <help_text> with a full stop for consistency.
-include(CMakeDependentOption)
 option(BUILD_DAEMON "Build bitcoind executable." ON)
 option(BUILD_CLI "Build bitcoin-cli executable." ON)
 option(BUILD_TX "Build bitcoin-tx executable." ON)
@@ -49,9 +52,9 @@ include(TristateOption)
 tristate_option(WITH_SQLITE "Enable SQLite wallet support." "if libsqlite3 is found." AUTO)
 tristate_option(WITH_BDB "Enable Berkeley DB (BDB) wallet support." "if libdb_cxx is found." AUTO)
 option(WARN_INCOMPATIBLE_BDB "Warn when using a Berkeley DB (BDB) version other than 4.8." ON)
+include(CMakeDependentOption)
 cmake_dependent_option(BUILD_WALLET_TOOL "Build bitcoin-wallet tool." ON "ENABLE_WALLET" OFF)
 
-cmake_dependent_option(CXX20 "Enable compilation in C++20 mode." OFF "NOT MSVC" ON)
 option(THREADLOCAL "Enable features that depend on the C++ thread_local keyword (currently just thread names in debug logs)." ON)
 option(HARDENING "Attempt to harden the resulting executables." ON)
 option(REDUCE_EXPORTS "Attempt to reduce exported symbols in the resulting executables." OFF)
@@ -71,14 +74,6 @@ option(BUILD_TESTS "Build test_bitcoin executable." ON)
 option(BUILD_BENCH "Build bench_bitcoin executable." ON)
 option(INSTALL_MAN "Install man pages." ON)
 
-if(CXX20)
-  set(CMAKE_CXX_STANDARD 20)
-else()
-  set(CMAKE_CXX_STANDARD 17)
-endif()
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
-
 set(configure_warnings)
 
 include(CheckPIESupported)

Changes from bitcoin#28894 have been backported as well.

---

What to test?

1. The minimum supported compilers.
2. Any fancy build setup mentioned in build: Require C++20 compiler bitcoin/bitcoin#28349.

[hebasto]

Friendly ping @TheCharlatan @fanquake @theuni @vasild @pablomartin4btc @m3dwards :)

[TheCharlatan]

LGTM ACK

[hebasto]

@TheCharlatan

Thank you for the reviewing!

---

This branch has been force pushed into the https://github.com/hebasto/bitcoin/tree/cmake-staging.

Closing.