cmake,options: Allow to set global client config file
This allows the global client (ssh_session) configuration file path to
be set in configuration time by defining GLOBAL_CLIENT_CONFIG when
calling cmake. If it is not defined, the default path is set as
$ cmake -DGLOBAL_CLIENT_CONFIG=/etc/my/custom/path ..
Signed-off-by: Anderson Toshiyuki Sasaki <email@example.com>
Reviewed-by: Andreas Schneider <firstname.lastname@example.org>