FindBoost: Fix Boost_LIBRARY_DIR_{RELEASE,DEBUG} cache entry configuration

These cache entries introduced by commit 892b854f (FindBoost: Search for
debug and release libraries separately, 2015-01-26) should be marked as
advanced just as Boost_LIBRARY_DIR was.  Also their _LAST values should
be tracked so changes can be detected reliably.  Both of these are
handled by code looking in _Boost_VARS_LIB for a list of relevant
variables.  Fix construction of this list that was broken by the above
commit.

Reported-by: Sylvain Joubert <joubert.sy@gmail.com>

Modules/FindBoost.cmake

@@ -879,8 +879,10 @@ endif()
 #  Begin finding boost libraries
 # ------------------------------------------------------------------------
 
+set(_Boost_VARS_LIB "")
 foreach(c DEBUG RELEASE)
   set(_Boost_VARS_LIB_${c} BOOST_LIBRARYDIR Boost_LIBRARY_DIR_${c})
+  list(APPEND _Boost_VARS_LIB ${_Boost_VARS_LIB_${c}})
   _Boost_CHANGE_DETECT(_Boost_CHANGE_LIBDIR_${c} ${_Boost_VARS_DIR} ${_Boost_VARS_LIB_${c}} Boost_INCLUDE_DIR)
   # Clear Boost_LIBRARY_DIR_${c} if it did not change but other input affecting the
   # location did.  We will find a new one based on the new inputs.