[CMake] Better handling of library dependencies for CPack

[Rolf Eike Beer]

> 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.

There are not that many binary package formats out there. Since we already know the absolute path of the libraries we depend on (since the find_package() things found them) we could feed them through "rpm -qf" or whatever meets that on Debian based systems. Then you automatically have the correct package names no matter how the distros decide to call them.

Eike