Math.gamma(Float::INFINITY) should return +INF

* math.c (tgamma): it seems that the implementaion of mswin has similar problem
  with mingw.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58685 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

math.c

@@ -778,7 +778,7 @@ math_erfc(VALUE unused_obj, VALUE x)
     return DBL2NUM(erfc(Get_Double(x)));
 }
 
-#if defined __MINGW32__
+#if defined _WIN32
 static inline double
 ruby_tgamma(const double d)
 {