cgraph: use GVDLL instead of _WIN32 to control storage-class attributes in cghdr.h

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

diff --git a/lib/cgraph/cghdr.h b/lib/cgraph/cghdr.h
index d6005612b6f3787f5e78477abc73d88932a2e455..b2c3cd0baf91135d9ae9b934895f4ca8cfcbed5a 100644 (file)
--- a/lib/cgraph/cghdr.h
+++ b/lib/cgraph/cghdr.h
@@ -12,7 +12,7 @@
 
 #include "config.h"
 
-#ifdef _WIN32
+#ifdef GVDLL
 #ifdef EXPORT_CGHDR
 #define CGHDR_API __declspec(dllexport)
 #else