FindBoost: Add system thread library to Boost_LIBRARIES
Add the system thread library to Boost_LIBRARIES when the boost thread component has been found.
The Boost::thread imported target already pulls in Threads::Threads. This changes does the same for projects using the Boost_LIBRARIES variable instead.
Background:
The Debian cmake package (unfortunately) is carrying a downstream patch for this problem. Original bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=563479
Topic-rename: FindBoost-link-threads
Edited by Brad King