Add std::isnan polyfill, fixing compilation under Embarcadero

Fixes #1438

include/internal/catch_polyfills.cpp

@@ -0,0 +1,31 @@
+/*
+ *  Created by Martin on 17/11/2017.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_polyfills.hpp"
+
+#include <cmath>
+
+namespace Catch {
+
+#if !defined(CATCH_CONFIG_POLYFILL_ISNAN)
+    bool isnan(float f) {
+        return std::isnan(f);
+    }
+    bool isnan(double d) {
+        return std::isnan(d);
+    }
+#else
+    // For now we only use this for embarcadero
+    bool isnan(float f) {
+        return std::_isnan(f);
+    }
+    bool isnan(double d) {
+        return std::_isnan(d);
+    }
+#endif
+
+} // end namespace Catch