do not include DLLMain into core when it is a static library (thanks to aglinarth for the patch)

modules/core/src/system.cpp

@@ -748,7 +748,7 @@ cvGetModuleInfo( const char* name, const char **version, const char **plugin_lis
         *plugin_list = plugin_list_buf;
 }
 
-#if defined CVAPI_EXPORTS && defined WIN32 && !defined WINCE
+#if defined OPENCV_BUILD_SHARED_LIB && defined CVAPI_EXPORTS && defined WIN32 && !defined WINCE
 BOOL WINAPI DllMain( HINSTANCE, DWORD  fdwReason, LPVOID )
 {
     if( fdwReason == DLL_THREAD_DETACH || fdwReason == DLL_PROCESS_DETACH )