configure.in: quite conftest

* configure.in (rb_cv_broken_backtrace): suppress extra output from
  conftest.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39367 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -2439,7 +2439,7 @@ void sigsegv(int signum, siginfo_t *info, void *ctx){
     void *trace[TRACE_SIZE];
     int n = backtrace(trace, TRACE_SIZE);
     if (n > 0) {
-	fprintf(stdout, "backtrace:%d\n",n);
+	/*fprintf(stdout, "backtrace:%d\n",n);*/
     } else {
 	abort();
     }