Fixed a wrong assumption in configure.in and Include/pyport.h. The is finite function is not called isfinite() but finite(). Sorry, my fault. :)
diff --git a/Include/pyport.h b/Include/pyport.h
index e375c39..225b126 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -405,8 +405,8 @@
  * macro for this particular test is useful
  */
 #ifndef Py_IS_FINITE
-#ifdef HAVE_ISFINITE
-#define Py_IS_FINITE(X) isfinite
+#ifdef HAVE_FINITE
+#define Py_IS_FINITE(X) finite
 #else
 #define Py_IS_FINITE(X) (!Py_IS_INFINITY(X) && !Py_IS_NAN(X))
 #endif