cmake: Warn about not encapsulated build properties

CMakeLists.txt

@@ -280,3 +280,17 @@ if(configure_warnings)
     endforeach()
     message("  ******\n")
 endif()
+
+# We want all build properties to be encapsulated properly.
+get_directory_property(global_compile_definitions COMPILE_DEFINITIONS)
+if(global_compile_definitions)
+  message(AUTHOR_WARNING "The directory's COMPILE_DEFINITIONS property is not empty: ${global_compile_definitions}")
+endif()
+get_directory_property(global_compile_options COMPILE_OPTIONS)
+if(global_compile_options)
+  message(AUTHOR_WARNING "The directory's COMPILE_OPTIONS property is not empty: ${global_compile_options}")
+endif()
+get_directory_property(global_link_options LINK_OPTIONS)
+if(global_link_options)
+  message(AUTHOR_WARNING "The directory's LINK_OPTIONS property is not empty: ${global_link_options}")
+endif()