Compilation fails when libargp is missing

commit a9d1cfa9 seems to have broken compilation when libargp or argp.h is missing, like is common on OSX

clang: warning: argument unused during compilation: '-fstack-clash-protection' [-Wunused-command-line-argument]
/Users/aris/git/libssh/examples/ssh_server.c:260:9: error: call to undeclared function 'set_default_keys'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        set_default_keys(sshbind,
        ^