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

Index: backend/snapscan-usb.c
--- backend/snapscan-usb.c.orig
+++ backend/snapscan-usb.c
@@ -72,6 +72,7 @@
 
 #include "snapscan-usb.h"
 #include "snapscan-mutex.c"
+#include <errno.h>
 
 #ifndef SHM_R
 #define SHM_R 0