CBD-697: Make compilation work with relative paths

Make the compilation work, even if the path in COUCH_SRC is
relative.

Change-Id: Ie2831fa114fd6b13989dd1d8ea4135b3b4a1054b
Reviewed-on: http://review.couchbase.org/22992
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>