[ci] Enable hb-freetype on Windows build tests

Make Uniscribe optional on nmake win32 builds

Most of harfbuzz clients don't need Uniscribe and DirectWrite support at all
as harfbuzz is a replacement for Uniscribe so Chromium and Firefox are using
harfbuzz without them on Windows.

This removal also helps win32 nmake clients to have reduced binary size and
libs count.

Fixes https://github.com/behdad/harfbuzz/issues/342

[ci] Whitelist more symbols and unbreak mac CI

Thanks to @khaledhosny, actually this is his work :)

Fixes https://github.com/behdad/harfbuzz/issues/336

Supercedes https://github.com/behdad/harfbuzz/pull/326
Fixes https://github.com/behdad/harfbuzz/pull/331

With this, I believe MATH table API is not ready to be merged.

test-ot-math: Use different scales for vertical and horizontal directions and add test for hb_ot_math_get_min_connector_overlap. (#327)

Note, the function now returns "half of horizontal advance width"
if top accent attachment for glyph is not explicitly defined.
This is what the spec requires.  Updated tests.