SelectLibraryConfigurations: Fix for cached <base>_LIBRARY

The line

  set( ${basename}_LIBRARY )

removes the normal variable, but if the corresponding cached variable is
present then line

  list( APPEND ${basename}_LIBRARY optimized "${_libname}" )

uses that and fails.  Replace the original line with

  set( ${basename}_LIBRARY "" )

to set the normal variable to empty instead of unsetting it.

Modules/SelectLibraryConfigurations.cmake

@@ -62,7 +62,7 @@ macro( select_library_configurations basename )
         # if the generator supports configuration types or CMAKE_BUILD_TYPE
         # is set, then set optimized and debug options.
         if( CMAKE_CONFIGURATION_TYPES OR CMAKE_BUILD_TYPE )
-            set( ${basename}_LIBRARY )
+            set( ${basename}_LIBRARY "" )
             foreach( _libname IN LISTS ${basename}_LIBRARY_RELEASE )
                 list( APPEND ${basename}_LIBRARY optimized "${_libname}" )
             endforeach()