Check if PRIO_MIN and PRIO_MAX are defined

The majority of the systems define PRIO_MAX and PRIO_MIN,
but there is an obscure system, whose name I am not going
to tell, where they were not defined.

renice.c

@@ -7,6 +7,14 @@
 
 #include "util.h"
 
+#ifndef PRIO_MIN
+#define PRIO_MIN -NZERO
+#endif
+
+#ifndef PRIO_MAX
+#define PRIO_MAX (NZERO-1)
+#endif
+
 static int
 renice(int which, int who, long adj)
 {