use 4.02.0+local-git-... rather than 4.02.0local+git-... by default

Thanks to Thomas Gazagnaire for the advice, which should solve the
issues that prompted FORCE_VERSION to be added in a much more
convenient way.
add base packages

For some unknown reason, the base-package specified in the
configuration file are not seen as installed by opam by default. After
    opam compiler-conf install
it is recommended to also run
    opam install base-unix base-bigarray base-threads
to force them to be explicitly present -- this will influence the
options opam passes to ./configure scripts for its packages.