BUILD: Disable dbus tests when running distcheck
    
    https://fedorahosted.org/sssd/ticket/2291
    
    The dbus tests that mock an sbus server were failing when make distcheck
    was ran by a user logged in through the SSSD.
    
    The reason was that the libtool wrapper around the test library alters
    the LD_LIBRARY_PATH and as a consequence, the standard getpwuid_r() calls
    the dbus server performs would load the in-tree NSS library and not the
    system one.  The-in tree library would then attempt to talk to an in-tree
    NSS socket, fail, which would fail the getpwuid_r call with an error such as:
    
    """
        Could not get password database information for UID of current process:
        User "???" unknown or no memory to allocate password entry
    """
    
    This patch adds a new configure-time option called --enable-dbus-tests
    that is enabled by default and disabled during distcheck. When the
    option is disabled, the tests that require a mocked dbus server are not
    compiled at all.