Sometimes users need to tweak the finding process of `libclang`
for `bindgen` via the `clang-sys`-provided environment variables.
Thus add a paragraph to the setting up guide, including a reference
to `clang-sys`'s relevant documentation.
Link: https://lore.kernel.org/rust-for-linux/CAKwvOdm5JT4wbdQQYuW+RT07rCi6whGBM2iUAyg8A1CmLXG6Nw@xxxxxxxxxxxxxx/
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
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