As of Boost 1.50 it is required to link Boost.System with CGAL. System
has been a Thread dependency all along and we simply never linked to
it and apparently got lucky.
Add it to the dependencies, add it in the manual and INSTALL file.
CGAL_Boost_USE_STATIC_LIBS is now an (advanced) CMake option on all
platforms, and is stored in the generated CGALConfig.cmake. That way, its
value is proposed as the default value for the same option when programs
using CGAL are configured using CMake.
The script UseCGAL includes the module CGAL_TweakFindBoost, to define
Boost_USE_STATIC_LIBS and Boost_ADDITIONAL_VERSIONS.