Avoid __dead macro on non-BSD to prevent reserved identifier warnings

Apple clang may warn about defining macros with reserved
names (double underscore prefix).  On non-BSD platforms,
define __dead as empty instead of __attribute__((noreturn))
since the noreturn hint is optional.

Author: renaudallard

thinproxy.c

@@ -66,8 +66,10 @@
 
 /* ---- portability ---- */
 
-#ifndef __dead
-#define __dead	__attribute__((__noreturn__))
+#ifdef __OpenBSD__
+/* __dead provided by <sys/cdefs.h> */
+#elif !defined(__dead)
+#define __dead	/* empty */
 #endif
 
 /*