modules/ssh/session (make-session): Fix libssh 0.8 support

* modules/ssh/session.scm (make-session): Don't try to set "parse-config?"
option when libssh version is older than 0.9.  Log a message and fallback to
the old Guile-SSH behavior instead (see
<#38>.)

modules/ssh/session.scm

@@ -46,6 +46,7 @@
 (define-module (ssh session)
   #:use-module (ice-9 optargs)
   #:use-module (ssh log)
+  #:use-module (ssh version)
   #:export (session
             session?
             %make-session
@@ -67,6 +68,9 @@
 (define-macro (session-set-if-specified! option)
   `(if ,option (session-set! session (quote ,option) ,option)))
 
+(define %libssh-minor-version
+  (string->number (cadr (string-split (get-libssh-version) #\.))))
+
 ;; This procedure is more convenient than primitive `%make-session',
 ;; but on other hand it should be a bit slower because of additional
 ;; checks.  I think we can put up with this. -avp
@@ -94,7 +98,17 @@ Return a new SSH session."
             (%gssh-session-parse-config! session #f))
            (else
             (throw 'guile-ssh-error "Wrong 'config' value" config))))
-        (session-set! session 'process-config? #f))
+        (if (>= %libssh-minor-version 9)
+            (session-set! session 'process-config? #f)
+            (format-log 'rare
+                        'make-session
+                        (string-append
+                         "process-config? option is not available"
+                         " (using libssh 0.~a.)"
+                         " Falling back to the old Guile-SSH behavior "
+                         " (no config setting.)"
+                         " See <https://github.com/artyom-poptsov/guile-ssh/issues/38>.")
+                        %libssh-minor-version)))
 
     (session-set-if-specified! port)
     (session-set-if-specified! user)