Use System HarfBuzz

[BwL1289]

Resolves #45.

Adds the ability to use system HarfBuzz and associated documentation.

[BwL1289]

@khaledhosny please let me know if this is acceptable.

CC @gorloffslava.

[anthrotype]

thanks for this PR!
it's not clear to me why should pypy 3.8 fail on the CI but 3.7 still work.. From the error message it might even be some bug in setuptools and/or pypy itself.
It doesn't seem like you made any changes to our own CI workflow, it's still only building the uharfbuzz module with vendored harfbuzz, which was working before this PR, so it's not clear to me why it would start failing now.
I just pushed an empty commit to this #202 (based off main) to see if it triggers there as well, without your changes.
Maybe we could also try to build with a more up to date PyPy version, if that works i'm happy to drop the ones that are currently failing.

[BwL1289]

@anthrotype awesome - thanks so much for the help on this. really appreciate it.

[anthrotype]

it looks like it failed with the same error so your PR is not the culprit
https://github.com/harfbuzz/uharfbuzz/actions/runs/10512034395/job/29124400429?pr=202#step:5:3264

I'd say let's not block this PR on the PyPy issue.

I was wondering, how easy or difficult would be to set up the CI to also test with USE_SYSTEM_LIBS=1? It'd be ok to only test this on linux

[BwL1289]

it looks like it failed with the same error so your PR is not the culprit https://github.com/harfbuzz/uharfbuzz/actions/runs/10512034395/job/29124400429?pr=202#step:5:3264

I'd say let's not block this PR on the PyPy issue.

Thank you!

I was wondering, how easy or difficult would be to set up the CI to also test with USE_SYSTEM_LIBS=1? It'd be ok to only test this on linux

I can put this in our backlog. We're juggling a lot with a small team (other contributions to tons of open source libs in addition to the platform we're building) so anything we can offload for now is greatly appreciated.

[anthrotype]

thanks, I just tested this locally and I can confirm it works. I'll open an issue for exercising the USE_SYSTEM_LIBS=1 on our CI as well.

[BwL1289]

Thanks @anthrotype

[anthrotype]

https://github.com/harfbuzz/uharfbuzz/releases/tag/v0.39.4

EDIT: arghh.. the upload to pypi failed. I had to tag a new release, sorry! v0.39.5 is gonna work