GNUmakefile: use correct dev/python path when installing bup

GNUmakefile: use correct dev/python path when installing bup

$(bup_python) no longer exists, we should use dev/python.
But since we change working directory, that needs a fully
qualified path.

Reported-by: Quaddle Me <quaddle.me@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
[rlb@defaultvalue.org: use $(CURDIR) instead of $(PWD); add . target dir]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>