> On which systems is O_APPEND not supported? It's part of the POSIX standard, and even Windows seems to have it.

That would be surprising.
An easy way to find this out would be to remove the ifdef:
352 #ifdef O_APPEND
353 if (append)
354 flags |= O_APPEND;
355 #endif

>> Whereas the behavior of O_APPEND causes an automatic seek to the end
>> before any write().
>
> True, but IIRC some systems seek on open() and some systems seek just before write().

POSIX makes it clear that only the later behavior is legal:
O_APPENDIf set, the file offset shall be set to the end of the file
prior to each write.