SelectLibraryConfigurations: Do not cache the _LIBRARY variable

SelectLibraryConfigurations module currently cache and mark as advanced
the variable ${basename}_LIBRARY.
${basename}_LIBRARY_RELEASE and ${basename}_LIBRARY_DEBUG are usually
cached, because they often come from find_library().
${basename}_LIBRARY on the other hand is always of type
"optimized;${${basename}_LIBRARY_RELEASE};debug;${${basename}_LIBRARY_DEBUG}"
or just "${basename}_LIBRARY_RELEASE" or "${basename}_LIBRARY_DEBUG" if
only one version of the library is not found, if both have the same
value, or if configuration types are not supported.

Caching and marking as advanced just ${basename}_LIBRARY_RELEASE and
${basename}_LIBRARY_DEBUG is enough, just by modifying these two
variables, the user has enough control on finding the library, and
having 3 variables is redundant and confusing.

Modules/SelectLibraryConfigurations.cmake

@@ -73,15 +73,11 @@ macro( select_library_configurations basename )
         endif()
     endif()
 
-    set( ${basename}_LIBRARY ${${basename}_LIBRARY} CACHE FILEPATH
-        "The ${basename} library" )
-
     if( ${basename}_LIBRARY )
         set( ${basename}_FOUND TRUE )
     endif()
 
-    mark_as_advanced( ${basename}_LIBRARY
-        ${basename}_LIBRARY_RELEASE
+    mark_as_advanced( ${basename}_LIBRARY_RELEASE
         ${basename}_LIBRARY_DEBUG
     )
 endmacro()