bpo-36020: Require vsnprintf() to build Python (GH-20899)

The C99 functions snprintf() and vsnprintf() are now required
to build Python.

PyOS_snprintf() and PyOS_vsnprintf() no longer call Py_FatalError().
Previously, they called Py_FatalError() on a buffer overflow on platforms
which don't provide vsnprintf().
4 files changed