I was a little concerned that Alex’s patch used “test A -ef B”, which is not specified by Posix. But it is apparently widely supported (and consider we would only need it for cross compilation). Anyway, I was going to suggest test "$(srcdir)" == "." instead, but maybe Xavier’s solution is slightly better again.

I say go with vpath.patch, perhaps with a brief comment explaining that the cp + mv dance avoids copying the file onto itself when doing an in-tree build.