GPO: Allow customization of GPO_CROND per OS
    
    For both Debian and Ubuntu, the cron PAM service is named "cron" instead
    of "crond", denying the use of the service by default.
    
    This patch enables the HAVE_$OS (HAVE_DEBIAN/HAVE_FEDORA...) macros to select
    the service name during build, allowing further customization if required.
    
    Resolves:
    https://bugs.launchpad.net/ubuntu/+source/sssd/+bug/1572908
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>