Silence clang errors about -Wformat-nonliteral due to missing intermediate attributes

By default, meson builds glib with -Werror=format=2, which implies -Werror=format-nonliteral. With these flags, clang errors out on e.g. the g_message_win32_error function, due to "format string is not a string literal". This function takes a format string, and passes the va_list of the arguments onwards to g_strdup_vprintf, which is annotated with printf attributes.

When passing a string+va_list to another function, GCC doesn't warn with -Wformat-nonliteral. Clang however does warn, unless the functions themselves (g_message_win32_error and set_error) are decorated with similar printf attributes (to force the same checks upon the caller) - see https://clang.llvm.org/docs/AttributeReference.html#format for reference.

Adding these attributes revealed one existing mismatched format string (fixed in the first commit).

However, this does seem to cause other warnings/errors with GCC, due to set_error ("") triggering error: zero-length gnu_printf format string [-Werror=format-zero-length] (revealed by CI).