cmake BUGFIX ssh_threads linking

Fixes #627
diff --git a/CMakeModules/FindLibSSH.cmake b/CMakeModules/FindLibSSH.cmake
index cdaae38..8ef0ac4 100644
--- a/CMakeModules/FindLibSSH.cmake
+++ b/CMakeModules/FindLibSSH.cmake
@@ -90,15 +90,15 @@
 
             if(LIBSSH_VERSION VERSION_LESS 0.8.0)
                 # libssh_threads also needs to be linked for these versions
-                string(REPLACE "libssh.so" ""
+                string(REPLACE "libssh.so" "libssh_threads.so"
                     LIBSSH_THREADS_LIBRARY
                     ${LIBSSH_LIBRARY}
                 )
-                string(REPLACE "libssh.dylib" ""
+                string(REPLACE "libssh.dylib" "libssh_threads.dylib"
                     LIBSSH_THREADS_LIBRARY
                     ${LIBSSH_THREADS_LIBRARY}
                 )
-                string(REPLACE "ssh.so" ""
+                string(REPLACE "ssh.so" "ssh_threads.so"
                     LIBSSH_THREADS_LIBRARY
                     ${LIBSSH_THREADS_LIBRARY}
                 )