Mark two public functions ZIP_EXTERN.
diff --git a/lib/zip_source_supports.c b/lib/zip_source_supports.c
index 3c100fb..2179037 100644
--- a/lib/zip_source_supports.c
+++ b/lib/zip_source_supports.c
@@ -44,7 +44,7 @@
 }
 
 
-zip_int64_t
+ZIP_EXTERN zip_int64_t
 zip_source_make_command_bitmap(zip_source_cmd_t cmd0, ...)
 {
     zip_int64_t bitmap;