cc -pthread   -Wall -Wextra -Werror -Wno-error=deprecated -Wno-error=deprecated-declarations math/M_PI_2l.c -o math/M_PI_2l -D_POSIX_C_SOURCE=202405L  -lm -lpthread   -liconv
math/M_PI_2l.c:8:26: error: use of undeclared identifier 'M_PI_2l'
    8 | long double  const foo = M_PI_2l;
      |                          ^~~~~~~
1 error generated.