win32 build

- force time_t to be 32 bits via Makefile. (If struct u gets changed to not
store a time_t field, this can be removed).

- Latest Windows SDK (supporting Vista) causes a warning due to
a prototype in sys/winnt/nttty. Since we don't actually link with that
function and rely on a DLL find procedure at run time, comment out
the prototype. The function is now officially in the SDK in the
windows header files anyway.

sys/winnt/nttty.c

@@ -1336,7 +1336,7 @@ adjust_palette(VOID_ARGS)
 
 /*
 /* only in Win2k+  (use FindWindow for NT4) */
-HWND WINAPI GetConsoleWindow();
+/* HWND WINAPI GetConsoleWindow(); */
 
 /*  Undocumented console message */
 #define WM_SETCONSOLEINFO			(WM_USER+201)