As suggested in #453 it is sometimes feasible to
select the backend bits via an override.
This change provides `cfg(curve25519_dalek_bits)`
to override the bits used in serial or fiat target backend.
The `RUSTFLAGS` were getting applied to build scripts, which caused them
to crash with SIGILL.
According to this issue, RUSTFLAGS won't be applied to build scripts
when cross-compiling by passing the `--target` attribute:
https://github.com/rust-lang/cargo/issues/4423
This attempts to work around the problem by explicitly passing:
--target x86_64-unknown-linux-gnu
As proposed in #414, this commit changes the backend selection approach,
introspecting `target_pointer_width` to select `u32_backend` vs
`u64_backend` (or `fiat_u32_backend`/`fiat_u64_backend` if the
`fiat_backend` feature is enabled).
This helps eliminate the use of non-additive features, and also the
rather confusing errors that happen if multiple backends are selected
(i.e. thousands of lines of rustc errors).
The selection logic checks if `target_pointer_width = "64"` and uses the
64-bit backend, or falls back to the 32-bit backend otherwise. This
means the crate will always have a valid backend regardless of the
pointer width, although there may be odd edge cases for exotic platforms
which would optimally use the 64-bit backend but have a non-"64" target
pointer width for whatever reason. We can handle those cases as they
come up.
This replaces the Travis CI workflows.
(cherry picked from commit d66a5a81f87dae29d0bb1a41bec26c57e6909c28)
Signed-off-by: Isis Lovecruft <isis@patternsinthevoid.net>
I discarded the portion of the original commit which removed TravisCI.
