make: unifdef for other OSes (undefined)

*** 42,54 ****
  #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.
--- 42,51 ----