add diagnostic when base C library (zenoh-c or zenoh-pico) is not available

[milyin]

Describe the feature

when zenoh-c and zenoh-cxx are added in wrong order:

find_package(zenohcxx)
find_package(zenohc)

The zenoh-cpp is actually doesn't define any target because it first checks for availability of zenoh-c or zenoh-pico.

Need to add diagnostics that neither zenoh-c nor zenoh-pico are found.