$ uname -smrv
Linux 5.1.17 #7 SMP Wed Jul 10 08:35:08 UTC 2019 aarch64

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.2 LTS
Release:        18.04
Codename:       bionic

$ dpkg -l|fgrep libssh
ii  libssh-4:arm64                       0.8.0~20170825.94fa1e38-1ubuntu0.2 arm64        tiny C SSH library (OpenSSL flavor)
ii  libssh-dev                           0.8.0~20170825.94fa1e38-1ubuntu0.2 arm64        tiny C SSH library. Development files (OpenSSL flavor)

$ ./configure
...
libssh support    yes
...

$ make
...
  CC      block/ssh.o
block/ssh.c: In function 'check_host_key_knownhosts':
block/ssh.c:281:28: error: storage size of 'state' isn't known
     enum ssh_known_hosts_e state;
                            ^~~~~
block/ssh.c:289:13: error: implicit declaration of function 'ssh_session_is_known_server' [-Werror=implicit-function-declaration]
     state = ssh_session_is_known_server(s->session);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
block/ssh.c:289:13: error: nested extern declaration of 'ssh_session_is_known_server' [-Werror=nested-externs]
block/ssh.c:293:10: error: 'SSH_KNOWN_HOSTS_OK' undeclared (first use in this function); did you mean 'SSH_OPTIONS_HOSTKEYS'?
     case SSH_KNOWN_HOSTS_OK:
          ^~~~~~~~~~~~~~~~~~
          SSH_OPTIONS_HOSTKEYS
block/ssh.c:293:10: note: each undeclared identifier is reported only once for each function it appears in
block/ssh.c:297:10: error: 'SSH_KNOWN_HOSTS_CHANGED' undeclared (first use in this function); did you mean 'SSH_KNOWN_HOSTS_OK'?
     case SSH_KNOWN_HOSTS_CHANGED:
          ^~~~~~~~~~~~~~~~~~~~~~~
          SSH_KNOWN_HOSTS_OK
block/ssh.c:301:48: error: 'SSH_PUBLICKEY_HASH_SHA256' undeclared (first use in this function); did you mean 'SSH_PUBLICKEY_HASH_SHA1'?
             r = ssh_get_publickey_hash(pubkey, SSH_PUBLICKEY_HASH_SHA256,
                                                ^~~~~~~~~~~~~~~~~~~~~~~~~
                                                SSH_PUBLICKEY_HASH_SHA1
block/ssh.c:307:27: error: implicit declaration of function 'ssh_get_fingerprint_hash'; did you mean 'ssh_get_pubkey_hash'? [-Werror=implicit-function-declaration]
             fingerprint = ssh_get_fingerprint_hash(SSH_PUBLICKEY_HASH_SHA256,
                           ^~~~~~~~~~~~~~~~~~~~~~~~
                           ssh_get_pubkey_hash
block/ssh.c:307:27: error: nested extern declaration of 'ssh_get_fingerprint_hash' [-Werror=nested-externs]
block/ssh.c:324:10: error: 'SSH_KNOWN_HOSTS_OTHER' undeclared (first use in this function); did you mean 'SSH_KNOWN_HOSTS_OK'?
     case SSH_KNOWN_HOSTS_OTHER:
          ^~~~~~~~~~~~~~~~~~~~~
          SSH_KNOWN_HOSTS_OK
block/ssh.c:329:10: error: 'SSH_KNOWN_HOSTS_UNKNOWN' undeclared (first use in this function); did you mean 'SSH_KNOWN_HOSTS_CHANGED'?
     case SSH_KNOWN_HOSTS_UNKNOWN:
          ^~~~~~~~~~~~~~~~~~~~~~~
          SSH_KNOWN_HOSTS_CHANGED
block/ssh.c:333:10: error: 'SSH_KNOWN_HOSTS_NOT_FOUND' undeclared (first use in this function); did you mean 'SSH_KNOWN_HOSTS_UNKNOWN'?
     case SSH_KNOWN_HOSTS_NOT_FOUND:
          ^~~~~~~~~~~~~~~~~~~~~~~~~
          SSH_KNOWN_HOSTS_UNKNOWN
block/ssh.c:337:10: error: 'SSH_KNOWN_HOSTS_ERROR' undeclared (first use in this function); did you mean 'SSH_KNOWN_HOSTS_OTHER'?
     case SSH_KNOWN_HOSTS_ERROR:
          ^~~~~~~~~~~~~~~~~~~~~
          SSH_KNOWN_HOSTS_OTHER
block/ssh.c:281:28: error: unused variable 'state' [-Werror=unused-variable]
     enum ssh_known_hosts_e state;
                            ^~~~~
cc1: all warnings being treated as errors
rules.mak:69: recipe for target 'block/ssh.o' failed
make: *** [block/ssh.o] Error 1