Document --threads, -j and throw an error when it's used on a non-threaded perl

lib/Slic3r/Config.pm

@@ -507,6 +507,12 @@ sub validate_cli {
 
 sub validate {
     my $class = shift;
+    
+    # -j, --threads
+    die "Invalid value for --threads\n"
+        if defined $Slic3r::threads && $Slic3r::threads < 1;
+    die "Your perl wasn't built with multithread support\n"
+        if defined $Slic3r::threads && !$Slic3r::have_threads;
 
     # --layer-height
     die "Invalid value for --layer-height\n"