Update after gnulib changed.
* autogen.sh: Create gettext-runtime/intl/attribute.h.
(GNULIB_MODULES_LIBASPRINTF): Add 'attribute'.
* gettext-runtime/intl/Makefile.am (EXTRA_DIST): Add attribute.h.
* Makefile.am (distcheck-hook): Test also gettext-runtime/intl/attribute.h.
* gettext-runtime/intl/vasnprintf.c: Update from gnulib.
* gettext-runtime/intl/xsize.h: Likewise.
* gettext-runtime/libasprintf/vasnprintf.c: Likewise.
