Merge bitcoin/bitcoin#33144: build: Set AUTHOR_WARNING on warnings

fa6497ba71 build: Set AUTHOR_WARNING on warnings (MarcoFalke)

Pull request description:

  Now that the cmake setting `-Werror=dev` is set since commit 6a13a6106e for the CI, guix and the dev cmake preset, it could make sense to notify developers about any warnings.

  So do that with a single `AUTHOR_WARNING`.

  This can be tested by introducing a bug, like:

  ```diff
  diff --git a/CMakeLists.txt b/CMakeLists.txt
  index 6017775fa7..5610e03c66 100644
  --- a/CMakeLists.txt
  +++ b/CMakeLists.txt
  @@ -589,7 +589,7 @@ set(Python3_FIND_FRAMEWORK LAST CACHE STRING "")
   # improves compatibility with Python version managers that use shims.
   set(Python3_FIND_UNVERSIONED_NAMES FIRST CACHE STRING "")
   mark_as_advanced(Python3_FIND_FRAMEWORK Python3_FIND_UNVERSIONED_NAMES)
  -find_package(Python3 3.10 COMPONENTS Interpreter)
  +find_package(Python3 3.210 COMPONENTS Interpreter)
   if(NOT TARGET Python3::Interpreter)
     list(APPEND configure_warnings
       "Minimum required Python not found."
  ```

  Fixes https://github.com/bitcoin/bitcoin/issues/31476.

ACKs for top commit:
  l0rinc:
    ACK fa6497ba71
  purpleKarrot:
    ACK fa6497ba71
  stickies-v:
    ACK fa6497ba71
  hebasto:
    ACK fa6497ba71, this effectively allows us to use `-Werror=dev` to automatically catch any warning.
  sedited:
    ACK fa6497ba71

Tree-SHA512: df3b7fa88451527d6a950bd6ebe46e96d1d2f6447c2b53cbe26c6ece0b63a41663a0accfc0ee20c03bc40328363d46f0e7ad88aab1be9b383ad7ff2621363a30

CMakeLists.txt

@@ -690,6 +690,7 @@ if(configure_warnings)
       message(WARNING "${warning}")
     endforeach()
     message("  ******\n")
+    message(AUTHOR_WARNING "Warnings have been encountered!")
 endif()
 
 # We want all build properties to be encapsulated properly.