It complains about being unable to determine the current SVN branch.
To fix it just change this line (55) from:
(my $branch) = ($url =~ m/\/svn\/webkit\/(trunk|branches\/[^\/]+)/);
(my $branch) = ($url =~ m/\/repository\/webkit\/(trunk|branches\/[^\/]+)/);
Now it works :)
That's not too surprising. I'm the only one that uses build-launcher-app, and I switched to using git long before we moved the SVN repository location.
I've never seen anything about git or how to build a .app with it...
Forward-duping to bug 20478 as that has a patch attached which fixes this issue as well as making build-launcher-app work inside a git repository.
*** This bug has been marked as a duplicate of 20478 ***