Fixes https://github.com/harfbuzz/harfbuzz/issues/345

appveyor.yml: try updating msys2 to fix failing mingw-w64-x86_64 build

maybe it'll get a bit faster

so it won't attempt reinstalling packages which are already installed

https://github.com/harfbuzz/harfbuzz/pull/1093#issuecomment-405201903
https://ci.appveyor.com/project/harfbuzz/harfbuzz/build/job/ky7lao1ii1bi7ew4#L71

It uses constexpr...

Also, include hb-dsalgs.hh from end of hb-pricate.hh

Oops.

Catches compile-errors in them better.

gen-def.py: pass headers as arguments so that msys2 can convert posix paths

let python's parse command-line args as usual

When one is not using the msys2 python, the header files that are passed in as environment
variable cannot be found.

https://ci.appveyor.com/project/fonttools/ttfautohint-py/build/1.0.65/job/rkremny4jjid9nl2#L803

This is because msys2 shell and make use POSIX paths (e.g. /c/Users/clupo/...)
whereas non-msys2 python.exe uses native Windows paths (e.g. C:\Users\clupo\...).

Msys2 will automatically convert command line arguments (but not environment variables) from
POSIX to Windows paths when calling a native win32 executable, so we pass the header paths
as arguments instead of environment variables.

This way the gen-def.py script can support both mingw python running in an MSYS2 shell, and
native win32 python.

Fixes #481

Completes #1037 and fixes #1035.

Disable vectorization for now.  To correctly use them, we should
use posix_memalign() to allocate them.  Otherwise, can cause
misaligned access.

https://bugs.chromium.org/p/chromium/issues/detail?id=860184

../../src/hb-ot-layout-gsubgpos-private.hh:391:18: warning: missed loop optimization, the loop counter may overflow [-Wunsafe-loop-optimizations]