Two-line fix for "./configure is too sensitive to stray files/subdirectories" #29120
Description
This branch can be merged by people who are working with tickets pre-9.1.beta2 who cannot easily merge current develop into that ticket.
It turns an error issued by configure when they are stray files in build/pkgs (such as the generated files introduced in #29038).
Before this ticket:
$ mkdir -p build/pkgs/xyzzy/foo
$ ./bootstrap && ./configure
...
configure: error: "/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/build/pkgs/xyzzy/type" is missing.
After this ticket:
...
configure: WARNING: "/Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/build/pkgs/xyzzy/type" is missing. Leftovers from another branch?
...
Component: build
Author: Matthias Koeppe
Branch/Commit: 171181f
Reviewer: Dima Pasechnik
Issue created by migration from https://trac.sagemath.org/ticket/29120