Define ATTRIBUTE_SENTINEL for GCC < 4.0 too

ATTRIBUTE_SENTINEL was defined for GCC >= 4.0 only, resulting in
compile errors when using GCC < 4.0.

src/internal.h

@@ -100,6 +100,8 @@
 #ifndef ATTRIBUTE_SENTINEL
 #if __GNUC_PREREQ (4, 0)
 #define ATTRIBUTE_SENTINEL __attribute__((__sentinel__))
+#else
+#define ATTRIBUTE_SENTINEL
 #endif
 #endif