thodg/libgit2

diff --git a/src/common.h b/src/common.h
index 5986a65..649c5d5 100644
--- a/src/common.h
+++ b/src/common.h
@@ -31,7 +31,9 @@
 
 # define snprintf _snprintf
 
+#ifndef _SSIZE_T_DEFINED
 typedef SSIZE_T ssize_t;
+#endif
 
 #else
 # include <unistd.h>