m4: checking if ssh_get_server_publickey() exists.

m4: checking if ssh_get_server_publickey() exists.

This commit adds some checks inside libssh m4 checking to verify if
ssh_get_server_publickey is available. This new function scope replaces
the old ssh_get_publickey() from libssh 0.7.5 and below. Assuming that
some distros are not showing the right version of libssh. This is a
simple way to check which function is available.

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>