Build broken when all -DWITH_XXX=system

cmake .. -DWITH_EDITLINE=system -DWITH_LIBEVENT=system -DWITH_LZ4=system -DWITH_MECAB=system -DWITH_SSL=system -DWITH_ZLIB=system

CMake Error at plugin/keyring/CMakeLists.txt:53 (ADD_COMPILE_FLAGS):
  Unknown CMake command "ADD_COMPILE_FLAGS".

Fix:
INCLUDE(${MYSQL_CMAKE_SCRIPT_DIR}/compile_flags.cmake)

(cherry picked from commit 70b6d76899732daf96da1f02777e4049dc2a6bca)