Use gcc4's visibility handling. Suggested by Tom Callaway.

--HG--
branch : HEAD

lib/zip.h

diff --git a/lib/zip.h b/lib/zip.h
index b98a060..48143a4 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -39,6 +39,8 @@
 #ifndef ZIP_EXTERN
 #ifdef _WIN32
 #define ZIP_EXTERN __declspec(dllimport)
+#elif defined(__GNUC__) && __GNUC__ >= 4
+#define ZIP_EXTERN __attribute__ ((visibility ("default")))
 #else
 #define ZIP_EXTERN
 #endif