Replace C style M_PI constant to C++20 std::numbers::pi in a few years

[innovation-plantation]

C++ standard finally defined pi. Compilers should catch up within a few years.

Once this feature becomes common (when the g++ compiler for the top distro on distrowatch defaults to support it),
insert the code:

#include <numbers>
using std::numbers::pi;

and rename M_PI to pi throughout Turtle.cpp

Until then, if your compiler no longer provides the old M_PI definition, you can define it on the compiler command line.

e.g. change g++ to g++ -DM_PI=3.14159 wherever g++ is called to compile the program.

[Flinner]

has the time come? :)