M_PI is not supposed to be defined in C99

[Xfennec]

The code is compiled with -std=c99 flag, but the C99 standard dropped M_PI and all other mathc defines of the same type. When compiling a compiler that strictly respects ISO like gcc, it generates a M_PI undefined error.

You should probably define your own PI, or compile with -std=gnu99 flag, for instance.

[coreh]

Thanks for pointing it out! Added my own.