Just add a line
#define BOOST_LOG_DYN_LINK 1
as the first line of boosttest.cc
.
Alternatively, you can add -DBOOST_LOG_DYN_LINK
to you compilation step (not the linking step, as you posted in the question):
g++ -std=c++11 -Wall -pedantic -g -O0 -DBOOST_LOG_DYN_LINK -c boosttest.cc
g++ boosttest.o -lpthread -lboost_log -o boosttest