Don't use $(wildcard) during 'make install'.

Don't use $(wildcard) during 'make install'.

It seems the $(wildcard) is evaluated once at make's startup, so any changes
made *during* build don't get noticed.

That means 'make install' would fail if you ran it without first running
'make all', because $(wildcard cmd/bup-*) wouldn't match anything at startup
time; the files we were copying only got created during the build.

Problem reported by David Roda.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>