* lib/sys_stat.in.h: Invoke _GL_INLINE_HEADER_BEGIN,
_GL_INLINE_HEADER_END. Include <errno.h>, <unistd.h>.
(_GL_ISSYMLINK_INLINE, _GL_ISSYMLINKAT_INLINE): New macros.
(issymlink, issymlinkat): New declarations.
* lib/unistd.in.h: Do the #include <fcntl.h>, when needed for O_CLOEXEC,
at the end of the file. So that when <fcntl.h> includes <sys/stat.h>,
the declarations of readlink() and readlinkat() on native Windows are
already present.
* lib/issymlink.c: New file.
* lib/issymlinkat.c: New file.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_REQUIRE_DEFAULTS): Initialize
GNULIB_ISSYMLINK, GNULIB_ISSYMLINKAT.
* modules/sys_stat-h (Depends-on): Add extern-inline.
(Makefile.am): Substitute GNULIB_ISSYMLINK, GNULIB_ISSYMLINKAT.
* modules/issymlink: New file.
* modules/issymlinkat: New file.