[Feature #20563] Update required Windows version

Update the default `NTVER`, so that the declarations of APIs
introduced since Windows 8 will be enabled.
https://learn.microsoft.com/cpp/porting/modifying-winver-and-win32-winnt

Author: nobu

win32/Makefile.sub

@@ -176,7 +176,7 @@ PLATFORM = mswin32
 !error RT not defined.  Retry from configure pass.
 !endif
 !ifndef NTVER
-NTVER = 0x0600
+NTVER = 0x0602 # _WIN32_WINNT_WIN8
 !endif
 !ifdef NTVER
 ARCHDEFS = -D_WIN32_WINNT=$(NTVER) $(ARCHDEFS)