BUILD: Change error message if missing cifsimap.h
    
    cifs-idmap plugin is enabled by default, but required header file cifsidmap.h
    needn't be available on other distributions. It was not clear that cifs-idmap
    plugin is optional feature of sssd. With this patch, configure will recommend
    to build sssd without cifs idmap plugin if cifsidmap.h is not available.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2125