2.99.8: (unstable)

* signal_impl:
  - Use std::weak_ptr<signal_impl> in connected slots,
  to avoid a memory leak.
  (Kjell Ahlstedt) Bug #775871
  - clear(): Really avoid erasing the slots prematurely.
  (Kjell Ahlstedt)
* slot_rep: Make destructor, destroy, and clone() virtual,
  instead of using function pointers that do the same thing.
  (Kjell Ahlstedt) Bug #777618

Build:
* Visual Studio builds: Require Visual Studio 2017.
  (Chun-wei Fan)
* Add tests/memleakcheck.sh, to run the tests under valgrind.
  (Kjell Ahlstedt) Bug #775871
* Make --disable-benchmark work.
  (Christophe Lermytte) Bug #774732
* Make silent builds really silent.
  (Kjell Ahlstedt) Bug #768797