Add auto-generation notices for generated files

This directs new devs to look for the source files instead of
wasting time editing generated files.

cmake/scripts/prepend_cppbessot_notice.cmake

@@ -0,0 +1,20 @@
+if(NOT DEFINED CPPBESSOT_TARGET_FILE OR "${CPPBESSOT_TARGET_FILE}" STREQUAL "")
+  message(FATAL_ERROR "CPPBESSOT_TARGET_FILE is required")
+endif()
+
+# Some CMake command invocations can pass wrapped quotes through -D values.
+set(_target_file "${CPPBESSOT_TARGET_FILE}")
+string(REGEX REPLACE "^\"(.*)\"$" "\\1" _target_file "${_target_file}")
+
+if(NOT EXISTS "${_target_file}")
+  message(FATAL_ERROR "Target file does not exist: ${_target_file}")
+endif()
+
+file(READ "${_target_file}" _content)
+
+if(_content MATCHES "AUTO-GENERATED BY CppBeSsot")
+  return()
+endif()
+
+set(_notice "/*\n * AUTO-GENERATED BY CppBeSsot.\n * Direct edits are pointless: this file is regenerated.\n * Update the OpenAPI schema and/or CppBeSsot templates, then regenerate generated-* outputs.\n */\n\n")
+file(WRITE "${_target_file}" "${_notice}${_content}")