make: unifdef for other OSes (undefined)

@@ -42,13 +42,10 @@
 #include <mksh/mksh.h>
 #include <mksdmsi18n/mksdmsi18n.h>
 #include <errno.h>
 #include <signal.h>
 
-#ifdef HP_UX
-        extern void (*sigset(int, void (*)(__harg)))(__harg);
-#endif
 
 /*
  * Workaround for NFS bug. Sometimes, when running 'chdir' on a remote
  * dmake server, it fails with "Stale NFS file handle" error.
  * The second attempt seems to work.