It specifies a directory or a list of directories in which to search for the MySQL library. If the value is a directory pathname designator then it is passed to directory. If the value is a list of directory pathname designators then each item is passed to directory. The collected results are the list of directories to search in.
The default value
nil causes the system to use *mysql-library-sub-directories* to construct the search path. With the default installation of MySQL this copes better with 64-bit/32-bit mixing on the same machine. When
*mysql-library-directories* is non-nil, it overrides *mysql-library-sub-directories*.
In LispWorks 6.1 and later,
*mysql-library-directories* has initial value
nil so the search path is constructed using *mysql-library-sub-directories*.
LispWorks User Guide and Reference Manual - 20 Sep 2017