build: Build para_client only if crypto lib was found.

This creates a separate section for para_client, similar to the one
for para_server that was added in the previous patch.

The new section sets $build_client to either yes or no, depending on
whether a crypto library (openssl nor gcrypt) was found by the tests
performed earlier. This test is always true at the moment since we
abort anyway if neither library was detected. But this is about to
change, so..