Added compiler attribute __unused__ for unused variables/functions.

include/sane/sane.h

@@ -20,6 +20,26 @@
 extern "C" {
 #endif
 
+/*
+ * Compiler related options
+ */
+
+#ifdef __GNUC__
+
+/* __unused tells the compiler a variable is unused, so the
+ * compiler doesn't spit a warning. */
+#define __unused__ __attribute__((unused))
+
+#else
+
+#define __unused__
+
+#endif
+
+/*
+ * SANE types and defines
+ */
+
 #define SANE_CURRENT_MAJOR	1
 
 #define SANE_VERSION_CODE(major, minor, build)	\