Win32: Open Watcom C compatibility fixes.

Win32: Open Watcom C compatibility fixes.

Precompiled headers are disabled as they lead to internal compiler errors
with long configure lines.  Couple of false positive warnings silenced.
Various win32 typedefs are adjusted to work with Open Watcom C 1.9 headers.

With this patch, it's now again possible to compile nginx using owc386,
with options we normally compile on win32 minus ipv6 and ssl.