Headers no correctly handle _XOPEN_SOURCE=700 (_POSIX_C_SOURCE=200809L)
Headers do not unhide many symbols when XPG7 level is requested by macros.
_XOPEN_SOURCE is processed by features_test.h correctly and _XPG7 is defined, but at least stdio.h and unistd.h have many commented out string like:
#if !defined(__XOPEN_OR_POSIX) || defined(_ATFILE_SOURCE) || \ defined(__EXTENSIONS__) /* || defined(_XPG7) */
#if defined(__EXTENSIONS__) || \ (!defined(_STRICT_STDC) && !defined(__XOPEN_OR_POSIX)) /* || defined(_XPG7) */
_XPG7 macro should be reviewed and uncommented.