Fix always rebuilding with --remap-path-prefix

[Nemo157]

Reapply https://github.com/rust-num/num-rational/pull/60 since that appears to have been reverted.

[cuviper]

Sorry about that! The build script was totally removed in #66, but then reintroduced in #56 where I didn't notice this aspect. With the relative order of these PRs, I guess it makes sense how that could be lost among rebases.

If you'd like to bump the crate version and add a quick release note, I can publish this fix as well.

[cuviper]

(wrong bors...)

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• Format
• No Std (1.31.0)
• No Std (stable)
• Test (1.31.0)
• Test (1.36.0)
• Test (beta)
• Test (nightly)
• Test (stable)