Update after gnulib changed.

Update after gnulib changed.

* autogen.sh: Don't copy attribute.h, as it would have the wrong copyright header.
* gettext-runtime/intl/attribute.h: New file, from gnulib.
* gettext-runtime/intl/arg-nonnull.h: Update from gnulib.
* gettext-runtime/intl/localename.c: Likewise.
* gettext-runtime/intl/vasnprintf.c: Likewise.
* gettext-runtime/intl/verify.h: Likewise.
* gettext-runtime/libasprintf/vasnprintf.c: Likewise.
* gettext-runtime/m4/fcntl-o.m4: Likewise.
* gettext-runtime/m4/iconv.m4: Likewise.
* gettext-runtime/intl/thread-optim.h: New file, from gnulib.
* gettext-runtime/intl/Makefile.am (EXTRA_DIST): Add thread-optim.h.
* Makefile.am (distcheck-hook): Check also thread-optim.h.