regint.h: version for secure functions

* regint.h (xvsnprintf): secure version functions are not
  supported on old VC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57175 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

regint.h

@@ -202,7 +202,7 @@
 #define xmemcpy     memcpy
 #define xmemmove    memmove
 
-#if defined(_WIN32) && !defined(__GNUC__)
+#if defined(RUBY_MSVCRT_VERSION) && RUBY_MSVCRT_VERSION >= 90 && !defined(__GNUC__)
 # define xalloca     _alloca
 # define xvsnprintf(buf,size,fmt,args)  _vsnprintf_s(buf,size,_TRUNCATE,fmt,args)
 # define xsnprintf   sprintf_s