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.
VS: Clarify Visual Studio product year for each version

Add to the brief documentation of the Visual Studio 10, 11, and 12
generators the corresponding VS product year.  Clarify that VS11 is for
Visual Studio 2012, and VS12 is for Visual Studio 2013.
Merge topic 'vs-generator-brief-doc-update'

52f1464 VS: Clarify Visual Studio product year for each version
Merge topic 'SelectLibraryConfigurations-cached-library'

c4bcc56 SelectLibraryConfigurations: Fix for cached <base>_LIBRARY