clause is silently ignored

When symbolic links are disabled by command line option or
NO_DIR_IN_CREATE sql mode, CREATE TABLE silently ignores
DATA/INDEX DIRECTORY options.

With this fix a warning is issued when symbolic links are disabled.


mysql-test/r/symlink.result:
  A test case for BUG#25677.
mysql-test/t/symlink.test:
  A test case for BUG#25677.
sql/sql_parse.cc:
  Moved handling of situation when mysqld is compiled without
  HAVE_READLINK to mysql_create_table_no_lock().
sql/sql_table.cc:
  Issue a warning in case DATA/INDEX DIRECTORY is specified and:
  - server is compiled without HAVE_READLINK;
  - using symbolic links is disabled by command line option;
  - using symbolic links is disabled by NO_DIR_IN_CREATE sql mode.