[CMake] Adding a custom target that depends on top-level `all' target.

[Eric Lemings]

Greetings,

If question is already answered somewhere, just point us to it.  I
couldn't find an answer in the CMake Wiki or FAQ.

How do I create a custom target in a subdirectory that is dependent on
the top-level target `all'?  The simple command `add_dependencies (foo
all)' doesn't work.

Thanks,
Eric.