[Configure] write single Makefile for multiple backends

does not work yet properly, because several targets are defined multiple times
[build] untagle build targets

you can now write "perl Configure.pl --backends=parrot,jvm"
and then "make p-all" and "make j-all", and it will build both
nqp-parrot and nqp-jvm (though the launchers might still be confused)
[build] unify "clean:" handling a bit