Always include errno.h . It is included through the elvin libs, but only

when eventsys is enabled, so the build was broken when events were
disabled (as they are in the testsuite).

pxe/proxydhcp.c

@@ -26,6 +26,7 @@
 #include <netdb.h>
 #include <string.h>
 #include <syslog.h>
+#include <errno.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/time.h>