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/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.
