CMakeLists.txt: Set policy CMP0151. (introduced in CMake-3.27)

clang complains about namespace pollution in generated moc_*.cpp files:

In file included from src/kde/kf5/kfilemetadata_rom-properties-kf5_autogen/mocs_compilation.cpp:2:
src/kde/kf5/kfilemetadata_rom-properties-kf5_autogen/BKZOEHIFDQ/moc_ExtractorPluginKF5.cpp:1082:17:
    warning: using namespace directive in global context in header [-Wheader-hygiene]
 1082 | using namespace RomPropertiesKF5;
      |                 ^

Setting CMP0151 tells AUTOGEN to add the include paths using -isystem,
which tells clang it shouldn't print warnings about these files.

CMakeLists.txt

@@ -3,12 +3,29 @@ CMAKE_MINIMUM_REQUIRED(VERSION 3.5)
 
 # CMP0048: Set VERSION variables based on the project version specified in PROJECT().
 # Introduced in CMake 3.0.
-CMAKE_POLICY(SET CMP0048 NEW)
+IF(POLICY CMP0048)
+	CMAKE_POLICY(SET CMP0048 NEW)
+ENDIF(CMP0048)
 
 # CMP0063: Honor visibility properties for all target types,
 # including static libraries and executables.
 # Introduced in CMake 3.3.
-CMAKE_POLICY(SET CMP0063 NEW)
+IF(POLICY CMP0063)
+	CMAKE_POLICY(SET CMP0063 NEW)
+ENDIF(POLICY CMP0063)
+
+# CMP0151: Use -isystem for AUTOMOC include directories.
+# clang complains about `using namespace` statements in the
+# generated moc_*.cpp files:
+# In file included from src/kde/kf5/kfilemetadata_rom-properties-kf5_autogen/mocs_compilation.cpp:2:
+# src/kde/kf5/kfilemetadata_rom-properties-kf5_autogen/BKZOEHIFDQ/moc_ExtractorPluginKF5.cpp:1082:17:
+#     warning: using namespace directive in global context in header [-Wheader-hygiene]
+#  1082 | using namespace RomPropertiesKF5;
+#       |                 ^
+# Introduced in CMake 3.27.
+IF(POLICY CMP0151)
+	CMAKE_POLICY(SET CMP0151 NEW)
+ENDIF(POLICY CMP0151)
 
 # Read the project version.
 CONFIGURE_FILE(version.txt version.txt.tmp)