CESNET
Support building a static library
This is literally the bare minimum that one has to do in order to let a properly configured CMake do its thing and produce a static library.
v2: be lazy and rely on CMake's builtin features
~~I'm deliberately not using CMake's standard way through the BUILD_SHARED_LIBS option because the other libraries in the NETCONF/YANG stack already stick to this custom convention.~~
Alright, seems like the right way to support this and we should update other projects this way, too. But please fix the conflicts before merging.
this will need some time on my end because we're currently stuck with the pre-v3/v4/whatever recent push (and we're testing some server-specific mock with actual communication samples, so that might take time)