https://github.com/harfbuzz/harfbuzz/issues/1546

Try at fixing https://github.com/harfbuzz/harfbuzz/issues/1546

Fixes https://github.com/harfbuzz/harfbuzz/issues/1308

Was good idea, but with C++ types with constructor/destructor, was getting in
the way as compiler was destructing those items where it was not desired.
Since C++ does not allow zero-sized arrays, just remove it...

Fixes https://github.com/harfbuzz/harfbuzz/issues/1491

Not needed.  We get it in our config.h automatically thanks to
AC_USE_SYSTEM_EXTENSIONS.  Let's see whose build it breaks...
If we end up putting it back, we should add other things from
that macro and remove the macro.

Since it was pages and pages...

That's better use of that value than requiring extra macro HB_NO_ATEXIT

Fixes #1235.

Cygwin is a Posix platform to the extent possible.  It should use the
Posix API except in special circumstances.

Disable copy/assign on them, as they shouldn't.

Make constructor / destructor call init_shallow/fini_shallow,
and make those idempotent.  So, these three can be constructed
on stack now and no init/fini call is needed.  As such,
hb_auto_t<> is not needed anymore.  I'll remove that separately.

Older C++ doesn't allow struct-with-constructor in union.