core fix: minor: suppressing VS 2015 warning 4275

VS15 is warning about dll-exporting a class without exporting its base class, which can be ignored for deriving from STL classes.

ext/cgt/exception.h

@@ -34,6 +34,10 @@
 #include <exception>
 #include <string>
 
+
+
+#pragma warning (disable: 4275) // VS15 warns about exporting a class derived from STL
+
 namespace cgt {
 
 /*
@@ -142,4 +146,7 @@ public:
 
 } // namespace cgt
 
+
+#pragma warning (disable: 4275)
+
 #endif // CGT_EXCEPTION