build: fix out-of-tree builds

Support for out-of-tree builds was added in
commit 3e4a9d60a9419621b08c647a306843d76c47c2cb
Author: Carl Worth <cworth@cworth.org>
Date:   Wed Mar 9 15:02:42 2011 -0800

    build: Add support for non-source-directory builds.

and broken in
commit 7beeb8c88a014ecbc53d8241f10683b3c4c16228
Author: David Bremner <bremner@debian.org>
Date:   Sat Nov 17 12:28:15 2012 -0400

    test: initial performance testing infrastructure

Fix the build breakage.

Out-of-tree 'make test' has been broken since earlier than the above,
and remains broken, as does out-of-tree perf test, but at least the
build now works.