static link into shared lib

----main.cpp
#include <iostream>

void doit();

int main()
{
        std::cout << "main\n";
        doit();
        return 0;
};

-------------------------
---example.cpp
#include <boost/thread.hpp>

void doit()
{
        boost::thread t1;
}
------------------------
---build script
g++ -Wall -fPIC -I/usr/include -c example.cpp -o example.o
g++ -shared /usr/lib/libboost_thread.a /usr/lib/libboost_system.a -Wl,-rpath,/usr/lib example.o -o libexample.so

g++ -Wall -c main.cpp -o main.o
g++ libexample.so main.o -o main
---------------------------

error message:
libexample.so: undefined reference to `boost::thread::thread()'
libexample.so: undefined reference to `boost::thread::~thread()'
collect2: ld returned 1 exit status