Use the u64 backend by default on x86_64 about curve25519-dalek HOT 3 CLOSED

One hack (mentioned in the Cargo issue and in the u128 tracking issue) is to use a #[cfg]s in a build script to emit feature selections for the main crate.

build.rs gets always compiled for the host platform but has to create programs for the target platform. If the two platforms are different (in a cross compilation scenario), the platform build.rs gets built for is different from the platform it has to build stuff for. So don't use #[cfg] or cfg! in build.rs for that purpose. Instead, build.rs has the TARGET env var.

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It seems like the "optimal" thing to do would be something like the following:

• Allow explicit backend selection with u32, u64, avx2 features;
• If no backend feature is specified, build.rs looks at the TARGET env var and makes a best-guess about which backend to use, then emits that feature for the main compilation.

However, because we load the entire crate into the build.rs already in order to generate constants using crate-private code, we need to have already selected the target backend before compiling the build script. So the approach above won't work. But this seems like the only way to access crate-private code without exposing it to the public.

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The current status is that the u64 backend is used by default, and that all backends are selectable with features.

Ideally we would auto-select the backend, but for the reasons described above we can't currently do this.

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