VS-2013 issue.

client/battle/CBattleInterface.cpp

@@ -34,11 +34,11 @@
 #include "../gui/CGuiHandler.h"
 #include "../CMT.h"
 
-#ifndef __GNUC__
-const double M_PI = 3.14159265358979323846;
-#else
+#if defined(_MSC_VER) && _MSC_VER >= 1800
 #define _USE_MATH_DEFINES
 #include <cmath>
+#else
+const double M_PI = 3.14159265358979323846;
 #endif
 #include "../../lib/UnlockGuard.h"