Autogen: Default AUTOGEN_USE_SYSTEM_INCLUDE to ON if it is not set

Add policy CMP0151 to preserve the old behavior by default.

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.27
 .. toctree::
    :maxdepth: 1
 
+   CMP0151: AUTOMOC include directory is a system include directory by default. </policy/CMP0151>
    CMP0150: ExternalProject_Add and FetchContent_Declare treat relative git repository paths as being relative to parent project's remote. </policy/CMP0150>
    CMP0149: Visual Studio generators select latest Windows SDK by default. </policy/CMP0149>
    CMP0148: The FindPythonInterp and FindPythonLibs modules are removed. </policy/CMP0148>

Help/policy/CMP0151.rst

@@ -0,0 +1,28 @@
+CMP0151
+-------
+
+.. versionadded:: 3.27
+
+AUTOMOC include directory is a system include directory by default.
+
+Headers generated for :ref:`Qt AUTOMOC` are placed in target-specific include
+directories.  CMake 3.26 and older added these as normal include directories.
+CMake 3.27 and newer prefer to add them as system include directories.
+This policy provides compatibility for projects that have not been updated
+to expect this.
+
+If the :prop_tgt:`AUTOGEN_USE_SYSTEM_INCLUDE` target property is set,
+perhaps via the :variable:`CMAKE_AUTOGEN_USE_SYSTEM_INCLUDE` variable,
+then its value is used regardless of the setting of this policy.
+
+The ``OLD`` behavior for this policy is to add autogen include directory to
+the target's include directories.
+The ``NEW`` behavior for this policy is to add autogen include directory to
+the target's system include directories.
+
+This policy was introduced in CMake version 3.27.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt

Help/prop_tgt/AUTOGEN_USE_SYSTEM_INCLUDE.rst

@@ -6,6 +6,9 @@ on a target to indicate that the autogen target include directory should
 be added as a system include directory or normal include directory to the
 target.
 
+If this property is not set, the autogen target include directory is added
+as a system include directory by default.  See policy :policy:`CMP0151`.
+
 See the :manual:`cmake-qt(7)` manual for more information on using CMake
 with Qt.