Revert part of PR#606 and use isnan/isinf again, but provide macro implementations of those in math_compat.h is needed, as it seems to be on AIX and IBM i systems.

json_object.c

@@ -903,15 +903,11 @@ static int json_object_double_to_json_string_format(struct json_object *jso, str
 	 * ECMA 262 section 9.8.1 defines
 	 * how to handle these cases as strings
 	 */
-#ifdef HAVE_DECL_ISNAN
 	if (isnan(jso->o.c_double))
 	{
 		size = snprintf(buf, sizeof(buf), "NaN");
 	}
-	else
-#endif
-#ifdef HAVE_DECL_ISINF
-	if (isinf(jso->o.c_double))
+	else if (isinf(jso->o.c_double))
 	{
 		if (jso->o.c_double > 0)
 			size = snprintf(buf, sizeof(buf), "Infinity");
@@ -919,7 +915,6 @@ static int json_object_double_to_json_string_format(struct json_object *jso, str
 			size = snprintf(buf, sizeof(buf), "-Infinity");
 	}
 	else
-#endif
 	{
 		const char *std_format = "%.17g";
 		int format_drops_decimals = 0;

math_compat.h

@@ -12,6 +12,9 @@
 #ifdef HAVE_DECL__ISNAN
 #include <float.h>
 #define isnan(x) _isnan(x)
+#else
+/* On platforms like AIX and "IBM i" we need to provide our own isnan */
+#define isnan(x) ((x) != (x))
 #endif
 #endif
 
@@ -19,6 +22,10 @@
 #ifdef HAVE_DECL__FINITE
 #include <float.h>
 #define isinf(x) (!_finite(x))
+#else
+#include <float.h>
+/* On platforms like AIX and "IBM i" we need to provide our own isinf */
+#define isinf(x) ((x) < -DBL_MAX || (x) > DBL_MAX)
 #endif
 #endif