Fixes https://github.com/harfbuzz/harfbuzz/pull/776

If we don't have visibility, make NullPool static.

Trying to fix:
https://circleci.com/gh/harfbuzz/harfbuzz/6635

Also part of:
https://github.com/harfbuzz/harfbuzz/issues/630

It's a hb_face_t that has add_table() and in the future knows how to
compile itself into a font blob.

Fixes https://github.com/harfbuzz/harfbuzz/issues/755

The atexit() man page indicates that this is expected to behave
in the expected way on unloading of shared libraries.

This fixes the build on Cygwin.

Hoping this would fix remaining text-rendering-tests failures,
but so far no luck.

Fixes https://github.com/harfbuzz/harfbuzz/issues/635

Part of fixing https://github.com/behdad/harfbuzz/pull/605

Might help with https://github.com/behdad/harfbuzz/pull/592
Ie. bringing in qsort_r() prototype always.

This concludes https://github.com/behdad/harfbuzz/pull/568

Even 1 is too many but putting 0 breaks compile.  Saves 3k in .so

Part of fixing https://github.com/behdad/harfbuzz/issues/585

Used to build static string arrays that use no relocation.

hb-ot-shape.cc:287:18: error: comparison of constant 32 with expression of type 'hb_unicode_general_category_t' is always true [-Werror,-Wtautological-constant-out-of-range-compare]

No flag is safe, over time... See previous commit.

Fixes https://github.com/behdad/harfbuzz/pull/23