mdds-2_1-2.1.1-bp156.1.1

- Relax requirement for compiler version, since gcc 7.x is able
  to compile C++17

- Update to 2.1.1
  * flat_segment_tree
    + added a method that returns a segment range object compatible
    with ranged for loop.
    + added a move constructor and a move assignment operator.
    + added variants of search() and search_tree() that return a
    result data structure that contains the value, the start and
    end keys of the range.
  * multi_type_vector
    + added a range adaptor for mdds::mtv::element_block compatible
    with ranged for loop.

- Build with gcc11 and gcc11-c++ on older distributions

- Initial Package (2.1.0)