* lib/sys_stat.in.h (stat) [GNULIB_OVERRIDES_STRUCT_STAT]: Provide own
definition. Define GNULIB_defined_struct_stat.
(fstat, fstatat, lstat, stat) [GNULIB_OVERRIDES_STRUCT_STAT]: Provoke
link error if this symbol is used and the corresponding module is not
in use.
(_stat64, _stat32i64, _stati64, _stat32, _stat64i32): Don't redefine if
GNULIB_OVERRIDES_STRUCT_STAT.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Initialize
GNULIB_OVERRIDES_STRUCT_STAT.
* modules/sys_stat (Makefile.am): Substitute
GNULIB_OVERRIDES_STRUCT_STAT.
* modules/windows-stat-override: New file.
