Merge pull request #15021 from andrey-golubev:gapi_fix_standalone_exports

* Fix G-API export specifier in standalone build

* Make dummy GAPI_EXPORTS in case of non-OpenCV builds

* Add old version under #if 0

modules/gapi/include/opencv2/gapi/own/exports.hpp

@@ -11,8 +11,10 @@
 #   if defined(__OPENCV_BUILD)
 #       include <opencv2/core/base.hpp>
 #       define GAPI_EXPORTS CV_EXPORTS
-
 #   else
+#       define GAPI_EXPORTS
+
+#if 0  // Note: the following version currently is not needed for non-OpenCV build
 #       if defined _WIN32
 #           define GAPI_EXPORTS __declspec(dllexport)
 #       elif defined __GNUC__ && __GNUC__ >= 4
@@ -22,6 +24,7 @@
 #       ifndef GAPI_EXPORTS
 #           define GAPI_EXPORTS
 #       endif
+#endif
 
 #   endif