Fixed various clang -Wself-assign warnings

Use idiomatic cast to void to indicate that a variable is unused.

libdispatch/ncexhash.c

@@ -285,6 +285,8 @@ exhashlocate(NCexhashmap* map, ncexhashkey_t hkey, NCexleaf** leafp, int* indexp
         /* Is there room in the leaf to add an entry? */
 #if DEBUG >= 3
 	fprintf(stderr,"locate: iter=%d offset=%x leaf=(%d)%p active=%d\n",iter,offset,leaf->uid,leaf,(int)leaf->active);
+#else
+	(void)iter;
 #endif
        if(leaf->active < map->leaflen) break; /* yes, there is room */
        /* Not Enough room, so we need to split this leaf */

libdispatch/ncjson.c

@@ -1140,5 +1140,5 @@ netcdf_supresswarnings(void)
     ignore = (void*)NCJtotext;
     ignore = (void*)NCJinsertstring;
     ignore = (void*)NCJinsertint;
-    ignore = ignore;
+    (void)ignore;
 }

libnczarr/zplugins.c

@@ -759,7 +759,7 @@ printhdf5(const NCZ_HDF5 h)
 static void static_unused(void)
 {
     void* p = NULL;
-    p = p;
+    (void)p;
     p = static_unused;
 #if defined(DEBUGF) || defined(DEBUGL)
 (void)printplugin(NULL);