MDEV-6565 search order for my.cnf inconsistent in docs/use, and global override with build-time -DDEFAULT_SYSCONFDIR is ignored

don't let INSTALL_SYSCONFDIR overwrite DEFAULT_SYSCONFDIR

CMakeLists.txt

@@ -337,7 +337,7 @@ ELSE()
 ENDIF()
 SET(DEFAULT_CHARSET_HOME "${DEFAULT_MYSQL_HOME}")
 SET(PLUGINDIR "${DEFAULT_MYSQL_HOME}/${INSTALL_PLUGINDIR}")
-IF(INSTALL_SYSCONFDIR)
+IF(INSTALL_SYSCONFDIR AND NOT DEFAULT_SYSCONFDIR)
   SET(DEFAULT_SYSCONFDIR "${INSTALL_SYSCONFDIR}")
 ENDIF()