Windows fixes from Kohei Yoshida:
- remove ZIP_EXTERN from functions not (yet?) part of public API
- provide fallback definition of ssize_t

Left to do:
- provide fallback definition of PRId64, PRIu64
- add getopt.c to regress programs when needed
diff --git a/lib/zip_source_read.c b/lib/zip_source_read.c
index 432243b..81b90ba 100644
--- a/lib/zip_source_read.c
+++ b/lib/zip_source_read.c
@@ -37,7 +37,7 @@
 
 
 
-ZIP_EXTERN zip_int64_t
+zip_int64_t
 zip_source_read(struct zip_source *src, void *data, zip_uint64_t len)
 {
     zip_int64_t ret;