Identify DLL platforms via CMAKE_IMPORT_LIBRARY_SUFFIX
Use CMAKE_IMPORT_LIBRARY_SUFFIX rather than checking windows to determine DLLPlatform (has import library) status.
This changes the definition of DLLPlatform per the initial description of #16801 (closed) to what was discussed there (using CMAKE_IMPORT_LIBRARY_SUFFIX) without touching the later discussion of CMAKE_PLATFORM_HAS_SONAME.
Fixes: #16801 (closed)
Edited by Brad King