BUILD: Make samba4 libraries optional
    
    Samba 4 libraries are necessary for building {ad, ipa} provider,
    but samba4 needn't be available on older distributions.
    This patch add possibility to build SSSD without {ad, ipa} provider
    and thus without Samba 4 libraries.
    The script configure have new argument --with-samba with default value yes.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>