/in/foo.c: In function 'main':
/in/foo.c:8:36: error: 'M_PI' undeclared (first use in this function)
     const double radians = 54.0 * (M_PI / 180.0);
                                    ^~~~
/in/foo.c:8:36: note: each undeclared identifier is reported only once for each function it appears in