TEST: Skip test_thread_exit if alert signal == 0

[mgondan]

No description provided.

[mgondan]

I open this for review.

1. It solves a problem in R package rswipl.
2. I am still a bit puzzled, though. Under linux, patch not included: test_thread_exit passes even if I invoke swipl --sigalert=0.

So, why does this work at all:

~/swipl/bin/swipl --sigalert=0 -f none --no-packs -g "use_module(library(http/th
read_httpd)), use_module(library(http/http_dispatch)), http_server(http_dispatch,[port(Port)]), http_stop_server(Port,[]
), prolog_alert_signal(Sig, Sig), writeln(sigalert=Sig)" -t "halt(1)"
% Started server at http://localhost:37973/
sigalert=0

..., given that http_stop_server uses thread_signal?