Compile-time error referencing M_PI when building on Ubuntu 18.04

examples/t04_blackboard.cpp uses the macro M_PI, which is not defined, resulting in:

```
src/behavior_tree_core/examples/t04_blackboard.cpp:44:34: error: ‘M_PI’ was not declared in this scope
     const Pose2D mygoal = {1, 2, M_PI};
                                  ^~~~
```
Perhaps add the following: 

```
#ifndef M_PI
#define M_PI <value>
#endif
```