Changes since 0.3.0:
- The x86 build now automatically falls back to "pure" Rust intrinsics,
under either of two possible conditions:
1. The `cc` crate fails to invoke a C compiler at all, indicating that
nothing of the right name (e.g. "cc" or "$CC" on Unix) is installed.
2. The `cc` crate detects that the compiler doesn't support AVX-512
flags, usually because it's too old.
The end result should be that most callers successfully build the
assembly implementations, and that callers who can't build those see a
warning but not an error. (And note that Cargo suppresses warnings for
non-path depencies.)