Add optional freetype support (Linux only)

[critiqjo]

The example provided may be re-written with purely freetype-sys as dependency, in which case it may even be included as a test in the harfbuzz-sys crate (though, an additional font file+licence would have to be added then).

EDIT: The module name hb_ft may not be appropriate...?

EDIT2: I'm not sure how this can be made to work in Windows since I'm dynamic linking freetype and that is not quite straightforward there.

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This change is

[critiqjo]

As an aside, I'm planning to write a higher-level wrapper for Harfbuzz (with lesser flexibility). Should I make a pull request here (since both harfbuzz-sys and harfbuzz crates are linked to this repo), or should I create it separately and would you agree to transfer me the harfbuzz ownership in crates.io (at least if it is good enough)?

EDIT: Now that I think about it, the versions in harfbuzz crate is not yanked. So it may still be used in which case transferring it to me would be a bad idea. On the other hand, I myself might want to assign a different name for the new crate, since it will not be meant to replicate all of harfbuzz' features & flexibility.

[critiqjo]

Ah! I realized this is a bad idea! And for my use-case, I'll just use an always-dynamically-linked fork of this so that freetype and harfbuzz are always inter-compatible (ensured by the system's package manager).