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 <a9e27ab562b8d02c0041eb70cde6fe3858219fb7@gmail.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

andreabolognani/libvirt,crobinso/libvirt,crobinso/libvirt,andreabolognani/libvirt,jardasgit/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,olafhering/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,eskultety/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,eskultety/libvirt,eskultety/libvirt,eskultety/libvirt,eskultety/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt