COMPAT: Set Prolog flag gc_thread to false

SWI-Prolog used to disable the garbage collection thread on `--no-signals`.
This was for JPL and is removed in version 9.1.18.   It is unclear why using
a separate GC thread causes the `prolog_in_java` test to hang.

This patch sets the GC thread explicitly.

src/main/c/jpl.c

@@ -3133,6 +3133,7 @@ jpl_do_pvm_init(JNIEnv *env)
   { msg = "jpl_do_pvm_init(): PL_initialise() failed";
     goto err;
   }
+  PL_set_prolog_flag("gc_thread", PL_BOOL, FALSE);
   /* *don't* free argv (must exist for lifetime of Prolog VM) */
 
   return jpl_post_pvm_init(env, argc, argv); /* TRUE, FALSE or exception */