CMake export targets properly

[rxdu]

Hi, the current CMake setup doesn't export the targets properly. It doesn't generate the mediasoupclientTargets.cmake and an external projects won't be able to find the libmediasoupclient project targets from outside easily. I was wondering if this kind of changes will be of interest to you. If yes, I would be glad to submit a pull request.

[ibc]

Yes, PR is welcome. It's just that we are terribly busy these months and cannot review/merge as quickly as we'd like.