allow overriding version-setting heuristic with FORCE_VERSION shell variable

Some package such as camlp4 explicitly test the ocaml version in ways
that are not compatible with the hand-crafted version names such as
4.02.0local generated by this tool. The new FORCE_VERSION variable
allows to override the ocaml-version of compiler-conf installs to make
these packages happy. Example:

  FORCE_VERSION=4.02.0 opam compiler-conf configure
  make world.opt -j5
  FORCE_VERSION=4.02.0 opam compiler-conf install
  opam install camlp4