Add Boost_REALPATH option for people packaging Boost with their app:

Boost_REALPATH               Resolves symbolic links for discovered boost libraries
                             to assist with packaging.  For example, instead of
                             Boost_SYSTEM_LIBRARY_RELEASE being resolved to
                             "/usr/lib/libboost_system.so" it would be
                             "/usr/lib/libboost_system.so.1.42.0" instead.
                             This does not affect linking and should not be
                             enabled unless the user needs this information.