Win32: Use the symbolic name for the default NTVER

And embed the given `_WIN32_WINNT` in config.h as well, for extension
libraries.

Author: nobu

win32/Makefile.sub

@@ -176,7 +176,7 @@ PLATFORM = mswin32
 !error RT not defined.  Retry from configure pass.
 !endif
 !ifndef NTVER
-NTVER = 0x0602 # _WIN32_WINNT_WIN8
+NTVER = _WIN32_WINNT_WIN8
 !endif
 !ifdef NTVER
 ARCHDEFS = -D_WIN32_WINNT=$(NTVER) $(ARCHDEFS)
@@ -664,6 +664,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 !if defined(ABI_VERSION)
 #define RUBY_ABI_VERSION $(ABI_VERSION)
 !endif
+#define _WIN32_WINNT $(NTVER)
 #define STDC_HEADERS 1
 #define HAVE_SYS_TYPES_H 1
 #define HAVE_SYS_STAT_H 1