Update from gnulib: Assume Autoconf >= 2.63.
* gettext-runtime/m4/po.m4 (AM_PO_SUBDIRS): Don't set localedir.
* gettext-runtime/m4/gettext.m4 (AM_GNU_GETTEXT): Update comment.
* gettext-runtime/m4/fcntl-o.m4 (gl_FCNTL_O_FLAGS): Assume
AC_USE_SYSTEM_EXTENSIONS exists.
* gettext-runtime/m4/threadlib.m4 (gl_THREADLIB_EARLY_BODY): Likewise,
* gettext-runtime/m4/longlong.m4: Require Autoconf >= 2.62. Update comments.
* gettext-runtime/m4/size_max.m4 (AC_COMPUTE_INT): Remove fallback for
Autoconf < 2.61.
