Depend on cppzmq-static when doing a static build.
cppzmq-static itself has a dependency problem with libsodium, so
explicitly set the missing but required libsodium dependency on it.
- Properly export the lokimq::lokimq target for use as a sub-project
- Drop the lokimq::static target; the library will just be static or
non-static dependending on cmake's BUILD_SHARED_LIBS setting
- Properly disable cppzmq tests (the previous setting resulted in cmake
warnings)