Fix duplicate symbols (see #787) since sdl2_ttf links against freetype

[Cheaterman]

I know, I know, it might look like a bit of a dumb solution... but we need freetype headers, and those won't be installed alongside sdl2_ttf ; so our best (easiest) solution is simply to make freetype not add its binary to the app, and have pillow rely on sdl2_ttf to provide the symbols instead while still relying on freetype for headers.

Of course if you have better suggestions, they're more than welcome! :-)

[misl6]

Mmm that seems quite hacky 😁

How about making the freetype symbols from sdl_ttf not visible instead? (or we can link sdl_ttf to our freetype lib, but we should make sure to keep both in sync)

[Cheaterman]

There's a few things :-) if we only hide the symbols, wouldn't we still potentially get issues when they go out of sync? Linking sdl2_ttf to our freetype lib sounds like a better idea in principle though, but I wasn't quite sure how to achieve it under my time constraints so I settled for this ; I'll try to improve it if/when I get time, otherwise at least we gave a few pointers for whoever feels up to the task :-)

[misl6]

Closed via #820