CMake: Avoid transitive linking of dependencies

By default, the dependencies of libiio would be appended to the linker
command line of iiod and the test programs. This resulted in the
Debian packages of iiod and libiio-utils having a direct dependency
towards Avahi or libxml2.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Debian: Fix Debian control file and build script, update .gitignore

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>