ocamlbuild: only show backtrace in verbose mode

Building foo.cmo in an empty directory (so with in particular no
foo.ml) currently returns the following output:

> Solver failed:
>   Ocamlbuild cannot find or build foo.ml.  A file with such a name
>   would usually be a source file.  I suspect you have given a wrong
>   target name to Ocamlbuild.
> Backtrace:
>   - Failed to build the target foo.cmo
>       - Building foo.cmo:
>           - Failed to build all of these:
>               - Building foo.ml:
>                   - Failed to build all of these:
>                       - Building foo.mly
>                       - Building foo.mll
>               - Building foo.mli:
>                   - Building foo.mly
>               - Building foo.mlpack
>               - Building foo.mli:
>                   - Building foo.mly
> Compilation unsuccessful after building 0 targets (0 cached) in 00:00:00.

While the "Solver failed" part is nice and reasonably easy to
understand, users report that the "Backtrace" part is confusing
(it talks about files they don't know about) -- and it can be so large
that the explanation above is completely hidden.

This patch disables backtrace-printing by default; it is now only
shown when some "-verbose N" (N starting at 1) argument is passed.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14732 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02