Minor refactorings for build.rs

[tesuji]

Use clone when appropriate.

[antonilol]

This fails to compile with bindgen and bundled features enabled, so please put the call to compute_include_paths back behind #[cfg(not(feature = "bundled"))].

I get that the formatting could be better, how about this instead?

#[cfg(feature = "bundled")]
let include_paths = vec![sdl2_includes];

#[cfg(not(feature = "bundled"))]
let include_paths = compute_include_paths(sdl2_includes);