diff --git a/git-compat-util.h b/git-compat-util.h
index af05077560..713f60d09d 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -185,7 +185,9 @@ struct strbuf;
 #define _ALL_SOURCE 1
 #define _GNU_SOURCE 1
 #define _BSD_SOURCE 1
+#ifndef _DEFAULT_SOURCE
 #define _DEFAULT_SOURCE 1
+#endif
 #define _NETBSD_SOURCE 1
 #define _SGI_SOURCE 1