error: use of undeclared identifier 'M_PI'

[yurivict]

/usr/ports/misc/iio-oscilloscope/work/iio-oscilloscope-0.17-master/oscplot.c:601:3: warning: implicit declaration of function 'gettimeofday' [-Wimplicit-function-declaration]
  601 |                 gettimeofday(&(priv->last_update), NULL);
      |                 ^
/usr/ports/misc/iio-oscilloscope/work/iio-oscilloscope-0.17-master/oscplot.c:899:20: error: use of undeclared identifier 'M_PI'
  899 |                 double a = 2.0 * M_PI / (n - 1);
      |                                  ^
/usr/ports/misc/iio-oscilloscope/work/iio-oscilloscope-0.17-master/oscplot.c:910:14: error: use of undeclared identifier 'M_PI'
  910 |                 double a = M_PI * j / (n - 1);
      |                            ^
/usr/ports/misc/iio-oscilloscope/work/iio-oscilloscope-0.17-master/oscplot.c:914:34: error: use of undeclared identifier 'M_PI'
  914 |                 return a0 - a1 * cos(j * 2.0 * M_PI / (n - 1));
      |                                                ^

Version: 0.17

[yurivict]

This is caused by -D_POSIX_C_SOURCE=200809L.
Removing it fixes the problem.

FreeBSD 14.1