[CMake] Better handling of library dependencies for CPack

[Rosen Diankov]

Hi all,

We've recently started generating (debian) packages with CPack and
discovered that handling library dependencies for particular
distributions was not as smooth as it can be. The *_DEPENDS string
just gets copied over to the final package so it puts all the burden
on the user side to manage what distribution's dependencies get
enabled.

Instead, can we have something similar to how build-type-dependent
libraries work out? For example, a library string can be set according
to:

set(LIBRARY debug mylibraryd optimized mylibrary)

And joining multiple library strings will take care of organizing the
dependencies. In the same way, it would be cool if we can specify
distribution dependencies that way:

set(*DEPENDS ubuntu mylib3-dev debian mylib-devel)

Then depending on the distribution type the user sets, the correct
dependencies can be selected.

rosen,