automatically fall back to the pure Rust build

There are two scenarios where compiling AVX-512 C or assembly code might
not work:

1. There might not be a C compiler installed at all. Most commonly this
   is either in cross-compiling situations, or with the Windows GNU
   target.
2. The installed C compiler might not support e.g. -mavx512f, because
   it's too old.

In both of these cases, print a relevant warning, and then automatically
fall back to using the pure Rust intrinsics build.

Note that this only affects x86 targets. Other targets always use pure
Rust, unless the "neon" feature is enabled.

1 files changed, 1 insertions, 3 deletions

diff --git a/README.md b/README.md
index 8f23a2b..2f6d04d 100644
--- a/README.md
+++ b/README.md
@@ -103,9 +103,7 @@ time b3sum /tmp/bigfile
 ### The `blake3` crate
 
 To use BLAKE3 from Rust code, add a dependency on the `blake3` crate to
-your `Cargo.toml`. Note that by default, unless the `pure` feature is
-enabled, building `blake3` requires a C compiler. Here's an example of
-hashing some input bytes:
+your `Cargo.toml`. Here's an example of hashing some input bytes:
 
 ```rust
 // Hash an input all at once.