This makes it possible to include all .cc files into build, even if not
building CoreText, Uniscribe, etc.

This was mostly to help custom builders.  But also means that we can
include all files in our own build system.  Not sure if we should.
Definitely simplifies things, but slightly only.

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Use of the deprecated API USCRIPT_CODE_LIMIT prevents harfbuzz
using the ICU4C as a shared library.

The API has been replaced by u_getIntPropertyMaxValue(UCHAR_SCRIPT)

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

Sorry for the noise, downstream custom builders.  Please adjust.

Add HB_USE_ATEXIT macros around free_static function definitions to
avoid unused function compiler warnings/errors.

Users don't expect a new reference returned from a get() function.
Indeed, all users of that API I foud where NOT destroying the reference.
Just change the implementations to NOT return a reference.

This applies to the following APIs:
hb_unicode_funcs_get_default()
hb_glib_get_unicode_funcs()
hb_icu_get_unicode_funcs()

Fixes https://github.com/harfbuzz/harfbuzz/issues/1134

We've had the proper barriers already.  No need for more with every access.

Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

This reverts commit 5daf3bd4.

If other atexit callbacks try to destruct the objects we destruct
in atexit callbacks, bad things will happen.

I'll come up with some other way to catch premature destruction
of HB-owned objects.

Fixes https://github.com/behdad/harfbuzz/issues/618

Such that client cannot accidentally destroy them, even though that
will be a bug in their code...

Oops.

The new unorm2_normalize has been public API since ICU 4.4, while
the old unorm_normalize has been deprecated since ICU 56.

Now, beginning with ICU 60, unorm_normalize will also be annotated
U_DEPRECATED and trigger the compiler -Wdeprecated-declarations warning.

Fix compile error in hb-icu.cc when ICU configured with
U_NO_DEFAULT_INCLUDE_UTF_HEADERS=1

As reported by Steven Loomis, including uversion.h works everywhere.