oasis should warn on potential overwrite of .o file

[ygrek]

Common (and very confusing) pitfall with bindings is having module.ml and module.c in same directory (ocaml and C side of bindings respectively) - which results in object file having same name for both inputs, and getting overwritten by ocamlbuild during build. The library will compile fine, but fail when linked with final program, because only one .o is linked.
Arguably this should be a warning/fix in ocamlbuild too, but warning in oasis would be very nice too and seems easier to implement.