intl: Simplify use of vasnprintf from Gnulib.
* autogen.sh (GNULIB_MODULES_LIBINTL): Use vasnprintf-posix instead of vasnprintf.
* gettext-runtime/intl/vasnprintf.h: Remove file, moved to gnulib.
* gettext-runtime/intl/Makefile.am (EXTRA_DIST): Remove vasnprintf.h.
(printf.lo): Simplify dependencies accordingly.
