This fix got lost in the shuffle.

M_PI is a non-standard constant that is sometimes defined in math.h. Let's define it as needed because it isn't standard and standard compilers are failing to compile gerbv when this is in use.

https://github.com/gerbv/gerbv/pull/368/changes#diff-3802cbcdb0af8c0a04ec0fc67cab4d18dbed5f686c2b6cad1d836323c0b64bbdR81-R90

#ifndef M_PI
# define M_PI		3.14159265358979323846	/* pi */
#endif
#ifndef M_PI_2
# define M_PI_2		1.57079632679489661923	/* pi/2 */
#endif
#ifndef M_1_PI
# define M_1_PI		0.31830988618379067154	/* 1/pi */
#endif