Issue with linking

[johanfforsberg]

(This is the thing I mentioned in the latest HDB++ meeting).

I was trying to build this project on my Ubuntu 20.4 machine for doing local testing and ran into some linker errors, complaining about missing "-lrt" flag. In the end I got it to work by adding rt to the target_link_libraries in CMakeLists.txt in both the hdbpp_es and hdbpp_cm subrepos. No real clue to why this is needed or if it's even the right approach.

I was compiling against the latest cpptango in conda-forge, which is 9.3.4.