lstat, openat: Respect rules for use of AC_LIBOBJ.

lstat, openat: Respect rules for use of AC_LIBOBJ.

* m4/lstat.m4 (gl_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK): Renamed from
AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK. Set variable
gl_cv_func_lstat_dereferences_slashed_symlink. Don't invoke AC_LIBOBJ.
(gl_PREREQ_LSTAT): New macro, extracted from gl_FUNC_LSTAT.
(gl_FUNC_LSTAT): Update. Remove gl_PREREQ_LSTAT code.
* modules/lstat (configure.ac): Invoke AC_LIBOBJ and gl_PREREQ_LSTAT
here.
* m4/openat.m4 (gl_FUNC_OPENAT): Update.