Description: really use XSI strerror_r.
  __EXTENSIONS__ enables _GNU_SOURCE on Dyson.
--- ffmpeg-3.2.5.orig/libavutil/error.c
+++ ffmpeg-3.2.5/libavutil/error.c
@@ -17,6 +17,7 @@
  */
 
 #undef _GNU_SOURCE
+#undef __EXTENSIONS__
 #define _XOPEN_SOURCE 600 /* XSI-compliant version of strerror_r */
 #include "avutil.h"
 #include "avstring.h"