add cdefs shim for defining __weak_alias on platforms without it

include/Makefile.am

@@ -1,6 +1,7 @@
 noinst_HEADERS =
 noinst_HEADERS += signal.h
 noinst_HEADERS += unistd.h
+noinst_HEADERS += sys/cdefs.h
 noinst_HEADERS += sys/queue.h
 noinst_HEADERS += sys/socket.h
 noinst_HEADERS += sys/types.h

include/sys/cdefs.h

@@ -0,0 +1,14 @@
+/*
+ * Public domain
+ * sys/cdefs.h compatibility shim
+ */
+
+#include_next <sys/cdefs.h>
+
+#ifndef __weak_alias
+/*
+ * We do not use any of the SHA function aliases that this would provide in
+ * OpenNTPD
+ */
+#define __weak_alias(alias,sym)
+#endif