Typo in testcpp.cc

[dsego]

Is this a typo? When I change to M_PI the build works, otherwise I get a build error.

https://github.com/mborgerding/kissfft/blob/8f47a67f595a6641c566087bf5277034be64f24d/test/testcpp.cc#L46

The error:

[ 96%] Building CXX object test/CMakeFiles/testcpp.dir/testcpp.cc.o
/Users/davorin/Code/projects/fft/kissfft/test/testcpp.cc:46:35: error: use of undeclared identifier 'M_PIl'; did you mean 'P_PID'?
        long double phinc = 2*k0* M_PIl / nfft;
                                  ^~~~~
                                  P_PID