Allow "classic" builds (= without InnoDB) by eliminating an "unresolved reference" (bug#7961).

sql/mysqld.cc:
  To allow builds without InnoDB ("classic"), the reference to 'innobase_use_checksums' must be guarded by an appropriate '#ifdef' (bug#7961).

sql/mysqld.cc

@@ -4449,9 +4449,11 @@ Disable with --skip-large-pages.",
 Disable with --skip-innodb (will save memory).",
    (gptr*) &opt_innodb, (gptr*) &opt_innodb, 0, GET_BOOL, NO_ARG, OPT_INNODB_DEFAULT, 0, 0,
    0, 0, 0},
+#ifdef HAVE_INNOBASE_DB
   {"innodb_checksums", OPT_INNODB_CHECKSUMS, "Enable InnoDB checksums validation (enabled by default). \
 Disable with --skip-innodb-checksums.", (gptr*) &innobase_use_checksums,
    (gptr*) &innobase_use_checksums, 0, GET_BOOL, NO_ARG, 1, 0, 0, 0, 0, 0},
+#endif
   {"innodb_data_file_path", OPT_INNODB_DATA_FILE_PATH,
    "Path to individual files and their sizes.",
    0, 0, 0, GET_STR, REQUIRED_ARG, 0, 0, 0, 0, 0, 0},