Restore change accidentaly reverted in previous: use _WIN32.

--HG--
branch : HEAD
diff --git a/lib/zip.h b/lib/zip.h
index f25dab2..0fae47b 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -37,7 +37,7 @@
 
 
 #ifndef ZIP_EXTERN
-#ifdef _MSC_VER
+#ifdef _WIN32
 #define ZIP_EXTERN __declspec(dllimport)
 #else
 #define ZIP_EXTERN