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.