clang -O2  -Wall -Wextra -Werror -Wno-error=deprecated -Wno-error=deprecated-declarations unistd/_SC_XOPEN_VERSION.c -o unistd/_SC_XOPEN_VERSION -D_POSIX_C_SOURCE=202405L  -lm  -lintl
unistd/_SC_XOPEN_VERSION.c:2:17: error: use of undeclared identifier '_SC_XOPEN_VERSION'
int const foo = _SC_XOPEN_VERSION;
                ^
1 error generated.