do not include libboost-context in the jar
- This is difficult because we may need to include libboost-chrono or
other libraries.
- This breaks the tests on some systems because of messages similar to
| OpenJDK Client VM warning: You have loaded library
| /tmp/simgrid-java7021501405032421762/libboost_context.so which might
| have disabled stack guard. The VM will try to fix the stack guard now.
- This make it difficult to build the jar on windows while allowing
parallel build.
So don't include the external libraries in the jar at all. We will
have to generate the native libraries with no dependencies for the
stable releases.