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