FindBoost: fix find_library call when using "re-rooting"

When using CMAKE_FIND_ROOT_PATH, FindBoost is able to find the first
component and cache the full path of the directory in Boost_LIBRARY_DIR
so that all components are looked for in same directory. The issue was
that, when looking for the other components, Boost_LIBRARY_DIR was
re-rooted against CMAKE_FIND_ROOT_PATH even though it was already a path
on the host. This change fixes this by disabling the re-rooting in the
find_library call when using Boost_LIBRARY_DIR as a hint.

See http://www.cmake.org/pipermail/cmake-developers/2014-October/011670.html
Merge topic 'find-boost-no-reroot' into next

43b8bfb2 FindBoost: fix find_library call when using "re-rooting"