cmake: Check for isblank().
SSH_AUTH_OK -> SSH_AUTH_SUCCESS in comments

A few callback descriptions refer to a non-existent value SSH_AUTH_OK,
which should be SSH_AUTH_SUCCESS.  This commit fixes these.

Signed-off-by: Alan Dunn <amdunn@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>