ocamlbuild testsuite: minor fixes

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14026 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ocamlbuild: fix plugin rebuild when no target is given

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14027 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
bootstrap myocamlbuild.boot and adapt build/boot.sh

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14028 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02