#define EXPORT __declspec(dllexport)
is defined in Host.h include from other public headers (SimpleSocket.h), with guard #ifdef _MSC_VER.
however this definition might collide easily with user code using CLsocket.
i'd suggest to rename EXPORT into CLSOCKET_EXPORT ..
[ and possibly make this definition only when having an additional private definition in CMakeLists.txt, e.g. when compiling the shared library ]