Oops. Need to check not only that HAVE_DECL_ISINF is defined, but also

that it's equal to 1.  (If isinf isn't defined, HAVE_DECL_ISINF is
defined to be 0, rather than being undefined.)

Include/pymath.h

@@ -87,7 +87,7 @@ extern double copysign(double, double);
  * Note: PC/pyconfig.h defines Py_IS_NAN as _isnan
  */
 #ifndef Py_IS_NAN
-#ifdef HAVE_DECL_ISNAN
+#ifdef HAVE_DECL_ISNAN && HAVE_DECL_ISNAN == 1
 #define Py_IS_NAN(X) isnan(X)
 #else
 #define Py_IS_NAN(X) ((X) != (X))
@@ -104,7 +104,7 @@ extern double copysign(double, double);
  * Note: PC/pyconfig.h defines Py_IS_INFINITY as _isinf
  */
 #ifndef Py_IS_INFINITY
-#ifdef HAVE_DECL_ISINF
+#ifdef HAVE_DECL_ISINF && HAVE_DECL_ISINF == 1
 #define Py_IS_INFINITY(X) isinf(X)
 #else
 #define Py_IS_INFINITY(X) ((X) && (X)*0.5 == (X))