missing.h: fix condition

* include/ruby/missing.h (bytesequence4_or_float): define by
  configured result.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52203 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

include/ruby/missing.h

@@ -136,7 +136,7 @@ RUBY_EXTERN double lgamma_r(double, int *);
 RUBY_EXTERN double cbrt(double);
 #endif
 
-#if !defined(INFINITY) || !defined(NAN)
+#if !defined(HAVE_INFINITY) || !defined(HAVE_NAN)
 union bytesequence4_or_float {
   unsigned char bytesequence[4];
   float float_value;