Support case-insensitive regex patterns

[gurry]

Builder methods allowlist_* and blocklist_* do not support case-insensitive regex patterns like ((?i).*foo) (note the (?i) modifier).

If you try to use them:

let bindings = bindgen::Builder::default()
        .header("myheader.h")
        .allowlist_file("(?i).*foo") // case-insensitive regex
        .generate()

you get errors like this:

regex parse error:
    ^((?i).*foo)$
            ^
error: Unicode-aware case insensitivity matching is not available (make sure the unicode-case feature is enabled)

As the error states, the unicode-case feature of the regex crate is required for such patterns. Unfortunately bindgen neither enables this feature nor expose a way to enable it.

A possible work around is for the user to to take a direct dependency on regex and configure the feature themselves. That works but is awkard. Ideally bindgen should provide native support.

To my knowledge this limitation is also not documented anywhere.