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

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

diff --git a/lib/cdt/cdt.h b/lib/cdt/cdt.h
index 034919bcd0722ff4765ced80d8644c0ca4334bfb..13e5837148bf486c2c772f2d2711368a7c7d9200 100644 (file)
--- a/lib/cdt/cdt.h
+++ b/lib/cdt/cdt.h
@@ -14,7 +14,7 @@ extern "C" {
 #include <stddef.h>    /* size_t */
 #include <string.h>
 
-#ifdef _WIN32
+#ifdef GVDLL
 #ifdef EXPORT_CDT
 #define CDT_API __declspec(dllexport)
 #else