Fixes https://github.com/harfbuzz/harfbuzz/issues/901

Fixes https://github.com/harfbuzz/harfbuzz/issues/901

For pre-C99 compilers that do not support round(), we need to have a
simplistic implementation for it, when it is not detected during build
configuration, either via CMake or autotools, by using floor() and
ceil(), which are provided in the pre-C99 compilers.

Please see discussion at commit 86a0ac28 for more details for re-adding
this patch.

This reverts commit b9dcbb1f.

This is gross. Should be only done if a specific macro is defined. Not the other way
around. This fails my build as autotools companions where not added anyway.

Add a simplistic round() implementation for our purposes, used when the
compiler does not support round() directly.

Should fix Win64 bot.

I'm confident that all bugs are hashed out now.

Compile all code-paths possible, to better catch errors.
Also enable MSVC ones on mingw which seems to support them.

Support 128bit type.

Visual C++ compiler doesn't define macros __OPTIMIZE__  when optimizations enabled

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.