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

README.markdown

@@ -85,7 +85,8 @@ The author is Alessandro Ranellucci (me).
         -o, --output <file> File to output gcode to (by default, the file will be saved
                             into the same directory as the input file using the 
                             --output-filename-format to generate the filename)
-        
+        -j, --threads <num> Number of threads to use (1+, default: 4) 
+      
       Output options:
         --output-filename-format
                             Output file name format; all config options enclosed in brackets

lib/Slic3r.pm

@@ -38,7 +38,8 @@ use Slic3r::Surface;
 use Slic3r::TriangleMesh;
 use Slic3r::TriangleMesh::IntersectionLine;
 
-our $threads            = 4;
+our $have_threads       = $Config{useithreads} && eval "use threads; use Thread::Queue; 1";
+our $threads            = $have_threads ? 4 : undef;
 
 # miscellaneous options
 our $notes              = '';
@@ -146,7 +147,7 @@ our $duplicate_distance = 6;    # mm
 sub parallelize {
     my %params = @_;
     
-    if (!$params{disable} && $Config{useithreads} && $Slic3r::threads > 1 && eval "use threads; use Thread::Queue; 1") {
+    if (!$params{disable} && $Slic3r::have_threads && $Slic3r::threads > 1) {
         my $q = Thread::Queue->new;
         $q->enqueue(@{ $params{items} }, (map undef, 1..$Slic3r::threads));
         

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"

slic3r.pl

@@ -95,6 +95,13 @@ if (@ARGV) {
 sub usage {
     my ($exit_code) = @_;
     
+    my $j = '';
+    if ($Slic3r::have_threads) {
+        $j = <<"EOF";
+    -j, --threads <num> Number of threads to use (1+, default: $Slic3r::threads)
+EOF
+    }
+    
     print <<"EOF";
 Slic3r $Slic3r::VERSION is a STL-to-GCODE translator for RepRap 3D printers
 written by Alessandro Ranellucci <aar\@cpan.org> - http://slic3r.org/
@@ -108,7 +115,7 @@ Usage: slic3r.pl [ OPTIONS ] file.stl
     -o, --output <file> File to output gcode to (by default, the file will be saved
                         into the same directory as the input file using the 
                         --output-filename-format to generate the filename)
-    
+$j
   Output options:
     --output-filename-format
                         Output file name format; all config options enclosed in brackets