Yves Frederix authored Jan 30, 2017 and Brad King committed Jan 30, 2017

This fixes a regression introduced by commit v3.3.0-rc1~5^2~2
(FindBoost: Search for debug and release libraries separately,
2015-01-26).  The `_Boost_CHANGE_LIBDIR` variable was split into
`_Boost_CHANGE_LIBDIR_{DEBUG,RELEASE}` but one usage site was not
updated.