Support generating an import depfile (.d) like GCC & Clang -MD option

When using the GN build system, we need to specify all imported emboss files (including recursively imported files) as inputs to the action target that runs embossc. It would be simpler and more maintainable to use a [depfile](https://gn.googlesource.com/gn/+/master/docs/reference.md#var_depfile) that was generated by emboss. The format of the depfile is a Makefile like the ones generated by GCC and Clang when the [-MD option](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-md) is used.

I haven't tried, but I think this format might work:
```
a.emb: b.emb c.emb
```