fix compilation without opencv

This PR fix master compilation if opencv2 is not found (#136 (closed)). @jsola can you try it to make sure that it still compiles if opencv2 is found ?

Note:
in file src/ceres_wrapper/create_auto_diff_cost_function.cpp, I use the preprocessor macro #ifdef CV_VERSION since I couldn't find another one more explicit such as CV_FOUND or alike.