FAQ says (for C++ compiler):
The constant M_PI is not supported. You have to define it yourself. For example, use const double PI = 2.0*acos(0.0).

This is not true. Such code is accepted:

#include <math.h>
#define _USE_MATH_DEFINES
...
printf ("%lf", M_PI);
...

May be FAQ should be corrected...

Edited by author 21.08.2009 17:57

ooops...

#define _USE_MATH_DEFINES
#include <math.h>
...