Don't link libmx and mxsh to the same libraries

Mxsh is linked to the same libraries as libmx itself. But they doesn't
necessary need the same things. For example libmx requires GnuTLS but
mxsh doesn't. It would be better if they were linked separately.

Link libmx and mxsh with just the libraries that they need, and not
anything else.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>