add support for system Bullet

[ognevny]

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[qwertychouskie]

Using system Bullet is not an option, see #5035 (comment)

[eli-schwartz]

This pull request doesn't contain any version matching to ensure that the correct version of bullet is used. In theory the objections from the linked ticket could could be answered via requiring specific versions or else mandating the bundled copy

While I would absolutely appreciate support for a system copy too, I think it's extremely important to collaborate with upstream's concerns that "just any version" will produce incorrect results.

[qwertychouskie]

This pull request doesn't contain any version matching to ensure that the correct version of bullet is used. In theory the objections from the linked ticket could could be answered via requiring specific versions or else mandating the bundled copy

While I would absolutely appreciate support for a system copy too, I think it's extremely important to collaborate with upstream's concerns that "just any version" will produce incorrect results.

Even "the exact same version" wouldn't work, as STK's Bullet has had modifications made over the years, including changes to functionality.

https://github.com/supertuxkart/stk-code/commits/master/lib/bullet