resolve undefined references

fully link internal dependencies, needed for creating dll - this is the same basic issue as https://github.com/ampl/mp/pull/6, found by building AMPL/MP with `-DBUILD_SHARED_LIBS:BOOL=ON -DCMAKE_SHARED_LINKER_FLAGS=-Wl,--no-undefined -DBUILD=all` on Ubuntu 14.04.

@vitaut it looks like you're the author of this cmake file, can you submit this upstream if you agree with the patch? I've never used gecode before but this seems like a safe change to me.