you need to link against libboost_system and apparently also against libboost_thread.
g++ -I /usr/local/boost_1_42_0 -lboost_system -lboost_thread a.cpp
in case of multi-threaded libraries:
g++ -I /usr/local/boost_1_42_0 -lboost_system-mt -lboost_thread-mt a.cpp