/* Deal with IEEE Std. 1003.1-1990, in which _POSIX_C_SOURCE == 1. */
  #if defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE == 1
  #undef _POSIX_C_SOURCE          /* Probably illegal, but beyond caring now. */
  #define _POSIX_C_SOURCE         199009
  #endif

cdefs.h, FreeBSD 10.1