It specifies a directory in which to search for the MySQL library, as a sub-directory of the appropriate Program Files directory. On a 32-bit machine that normally means
C:\Program Files\, while on a 64-bit machine it normally means
C:\Program Files\ for 64-bit programs and
C:\Program Files (x86)\ for 32-bit programs.
The value must be a pathname designator. It is merged with the Program Files directory yielding a path (for example
"C:\\Program Files\\MySQL\\MySQL*\\bin") which is then passed to directory. The result is a list of directories that are used to search for the MySQL library.
If *mysql-library-directories* is non-nil, it overrides
Note that this default will match any MySQL release, so if you need to be sure to match a specific MySQL release, you need to change the value of
*mysql-library-sub-directories* such that it matches only that particular release.
LispWorks User Guide and Reference Manual - 20 Sep 2017