devel: add script to test out-of-tree builds

Something I used for 'git bisect run', but we should really add this
as part of our process.

devel/out-of-tree-build-check.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+# test out-of-tree builds in a temp directory
+# passes all args to make
+
+set -eu
+
+srcdir="$(cd "$(dirname "$0")"/.. && pwd)"
+builddir=$(mktemp -d)
+
+cd $builddir
+
+$srcdir/configure
+make "$@"
+
+rm -rf $builddir