`#if FOO` (where FOO is undefined) is a legal construct in C89 and C99
but gcc, clang and sparse complain loudly about it at higher warning
levels.
Squelch those warnings. Makes the code more consistent as well.
accept4() piggybacks on the socketcall() on i386. socketcall() has the flaw
that it returns EINVAL instead of ENOSYS when the operation is not supported.
The problem is that accept4() also returns EINVAL when its flag argument is
invalid.
Try to discern between the two failure cases to the best of our abilities.
