[cpp] MSVC doesn't understand constant.

spine-cpp/spine-cpp/include/spine/MathUtil.h

@@ -37,10 +37,10 @@
 #include <string.h>
 
 namespace spine {
-float PI = 3.1415926535897932385f;
-float PI_2 = PI * 2;
-float DEG_RAD = (PI / 180.0f);
-float RAD_DEG = (180.0f / PI);
+static const float PI = 3.1415926535897932385f;
+static const float PI_2 = PI * 2;
+static const float DEG_RAD = (PI / 180.0f);
+static const float RAD_DEG = (180.0f / PI);
 
 class MathUtil : public SpineObject {
 private: