Updated VC++ patch

Here's an updated version of the VC++ patch. Changes since last time:

* Moved _dosmaperr() to port (from backend/port), and use it in
dirent.c, per Qingqing. (so backend/port/win32/error.c should be deleted
now). Please double-check the configure.in change, because my autoconf
just won't cooperate on win32 :-(
This also adds a couple of #ifdef FRONTEND for the ereport calls.

* Change definition of NAN, per Tom.

* Change src/port/thread.c so it doesn't pull in pthread.h when building
the backend, since it doesn't use any thread-specific functions there
anyway.

Tom has already committed a fix for the other float8 problem.

//Magnus