gvc++: use GVDLL instead of _WIN32 to control storage-class attributes in GVLayout.h

Towards https://gitlab.com/graphviz/graphviz/-/issues/2173.

diff --git a/lib/gvc++/GVLayout.h b/lib/gvc++/GVLayout.h
index b961824da182bd8ddb12a4a62ebbbcb1f20f8333..9de45440b4c148dca7eb35328c72add980418522 100644 (file)
--- a/lib/gvc++/GVLayout.h
+++ b/lib/gvc++/GVLayout.h
@@ -6,7 +6,7 @@
 #include "GVRenderData.h"
 #include <cgraph++/AGraph.h>
 
-#ifdef _WIN32
+#ifdef GVDLL
 #if gvc___EXPORTS // CMake's substitution of gvc++_EXPORTS
 #define GVLAYOUT_API __declspec(dllexport)
 #else