avoid linking with cmocka when not required
avoid linking with cmocka when not required

Only a few unit tests require to link with the cmocka library. A mistake
in the configure.ac made the -lcmocka flag global. This change ensures
that the link flag is used on unit tests only.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc