Clear up some build issues when used as subproject

Inconvenient to get all the dependencies in there (platform, cbcrypto, cbsasl, and all the third party libs). In particular, some of the third party libs really are public dependencies (headers depend on headers in that lib), maybe some are private (only .cxx depend on them), and worth making explicit and easy to use.

Ideally, you'd be able to target_link_libraries(<your target> ${<something like couchbase-cxx-client-libraries>}) and that's that. We could make ${COUCHBASE_CXX_CLIENT_INCLUDES} and ${COUCHBASE_CXX_CLIENT_LIBS} variables, and use them.

Lets make it simpler now, but we can always revisit later as this becomes its own stand-alone sdk.