CMake: Make programs link to libiio.so.0 instead of libiio.so.0.2

That way, we can increment the minor number of the version of the
library, and have all applications use the new library without having to
recompile them.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
CMake: Add a "doc" target to generate documentation with Doxygen

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