errno.h header is required now that errno is a per-thread variable.
"undefined symbol 'errno'" at runtime with dlopen()'d modules.

Index: backend/pint.c
--- backend/pint.c.orig
+++ backend/pint.c
@@ -42,7 +42,7 @@
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>
-extern int errno;
+#include <errno.h>
 
 #include "../include/sane/sane.h"
 #include "../include/sane/saneopts.h"