autotools: use C99 printf format specifiers on Windows. Fixes #1497
Since we now require a C99 compatible printf and use gnulib on Windows, we also mark our printf functions as gnu_printf. GCC complains about the Windows specific I64 specifiers we still write to glibconfig.h with the autotools build.
To fix this switch all I64(x) to ll(x).
This also makes the glibconfig.h output for those macros match the ones we get when using meson.