Since 4.03, OCaml supports coloring its messages to standard output and standard
error, depending on the "-color" argument ({always,never,auto}). This commit
adds support for the environment variable "OCAML_COLOR" (which value can as well
be {always,never,auto}).
The command line argument "-color" takes precedence, "OCAML_COLOR" is only
taken into consideration if no "-color" is provided.
The motivation for this is that the user should have control over coloring
OCaml's output messages. OCamlbuild, a widely used build tool executes OCaml
not under a tty (and OCaml does not colorize errors and warnings), which lead
various packages use `color(always)` in their `_tags` files, which breaks with
other (non-interactive) programs (i.e. editor helpers).
Further discussion was done at https://github.com/ocaml/ocamlbuild/issues/87 and
https://github.com/ocaml/ocaml/pull/1098.
76bf7c568e