AIX: Restore pre-3.16 undocumented method to suppress exports with XL
In !3556 (merged) we dropped use of the old CMAKE_XL_CreateExportList
cache entry for XL exports. However, some people were setting the value to an empty string as a way to disable automatic export of symbols. Restore this behavior when the option is explicitly set to an empty string.
Issue: #20290 (closed)
Backport: release
Edited by Brad King