stdio: Avoid GCC >= 4.4 warnings when using %lld and similar on mingw.

stdio: Avoid GCC >= 4.4 warnings when using %lld and similar on mingw.

* lib/stdio.in.h (_GL_ATTRIBUTE_FORMAT_PRINTF): Use 'gnu_printf' format
instead of 'printf' format for GCC >= 4.4.
(_GL_ATTRIBUTE_FORMAT_PRINTF_SYSTEM): New macro.
(fprintf, printf, vfprintf, vprintf): Declare with
_GL_ATTRIBUTE_FORMAT_PRINTF_SYSTEM when the function ends up calling
the system's vfprintf() function.
Reported by Daniel P. Berrange <berrange@redhat.com> via Eric Blake.