Add build.rs to messages to automatically find the message libraries

[nnmm]

I think the relative path will be always correct, but even if not, this is only an additional search dir.

The impact is that you can build packages depending on message packages much more easily with cargo.

[esteve]

@nnmm can you elaborate a bit more what issue this PR fixes? Thanks.

[nnmm]

@esteve Even if you have a .cargo/config.toml so that dependencies can be found, you cannot currently build packages depending on messages with cargo. You also need to source the install beforehand, because otherwise you'll get a linker error about not finding librclrs_example_messages_typesupport.so or somesuch.

This build script fixes that, so that building with pure cargo becomes easier.

[esteve]

@nnmm thanks for the explanation, makes sense.

[esteve]

Is this expecting a specific directory structure? What would happen with a merge install? i.e. everything installed into the install directory, no separate packages.

[nnmm]

Yep, the relative path is still the same. I tested it.