diff -r 000000000000 -r 6474c204b198 build/unix/abs2rel.pl
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/build/unix/abs2rel.pl	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,35 @@
+#!env perl
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+use File::Spec::Unix;
+use strict;
+
+print "Usage: $0 dest_path start_path\n" if ($#ARGV+1 != 2);
+my $finish = my_canonpath(shift);
+my $start = my_canonpath(shift);
+
+my $res = File::Spec::Unix->abs2rel($finish, $start);
+
+#print STDERR "abs2rel($finish,$start) = $res\n";
+print "$res\n";
+
+sub my_canonpath($) {
+    my ($file) = @_;
+    my (@inlist, @outlist, $dir);
+
+    # Do what File::Spec::Unix->no_upwards should do
+    my @inlist = split(/\//, File::Spec::Unix->canonpath($file));
+    foreach $dir (@inlist) {
+	if ($dir eq '..') {
+	    pop @outlist;
+	} else {
+	    push @outlist, $dir;
+	}
+    }
+    $file = join '/',@outlist;
+    return $file;
+}
+