Ensure superbuild version of Boost is found

The Boost_USE_MULTITHREADED option needs to be turned off
when using the superbuild version of Boost. This tells
FindBoost to look for the non threaded version. Otherwise a
system threaded version may be found instead if its installed.

Change-Id: I5410858a9fdd190135baf6da857500a6da987732
Merge topic 'boost-fix' into master

7a201e62 Ensure superbuild version of Boost is found