Dictzip only really works in the threaded code paths. So let's force
use them and ignore the non-parallel case (bnc#597756).

Index: pigz-2.1.5/pigz.c
===================================================================
--- pigz-2.1.5.orig/pigz.c
+++ pigz-2.1.5/pigz.c
@@ -2820,7 +2820,7 @@ local void process(char *path)
             unlzw();
     }
 #ifndef NOTHREAD
-    else if (procs > 1)
+    else if (1)
         parallel_compress();
 #endif
     else