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.

src/ceres_wrapper/create_auto_diff_cost_function.cpp

@@ -20,7 +20,11 @@
 #include "../constraint_point_2D.h"
 #include "../constraint_point_to_line_2D.h"
 #include "../constraint_container.h"
-#include "../constraint_AHP.h"
+
+#ifdef CV_VERSION
+  #include "../constraint_AHP.h"
+#endif
+
 #include "../constraint_imu.h"
 
 // Wolf and ceres auto_diff creators
@@ -98,7 +102,7 @@ ceres::CostFunction* createAutoDiffCostFunction(ConstraintBasePtr _ctr_ptr, bool
             	return createAutoDiffCostFunctionWrapper<ConstraintPointToLine2D>(_ctr_ptr);
             else
                 return createAutoDiffCostFunctionCeres<ConstraintPointToLine2D>(_ctr_ptr);
-
+#ifdef CV_VERSION
         case CTR_EPIPOLAR:
             if (_use_wolf_autodiff)
                 return createAutoDiffCostFunctionWrapper<ConstraintAHP>(_ctr_ptr);
@@ -110,7 +114,7 @@ ceres::CostFunction* createAutoDiffCostFunction(ConstraintBasePtr _ctr_ptr, bool
                 return createAutoDiffCostFunctionWrapper<ConstraintAHP>(_ctr_ptr);
             else
                 return createAutoDiffCostFunctionCeres<ConstraintAHP>(_ctr_ptr);
-
+#endif
         case CTR_IMU:
             if (_use_wolf_autodiff)
                 return createAutoDiffCostFunctionWrapper<ConstraintIMU>(_ctr_ptr);