Added CMake support

[SteveGremory]

I needed to use BLAKE3 as a CMake submodule and luckily someone had done the work for me.

This PR makes BLAKE3 usable as a CMake submodule and helps with rather easy compilation of the library across platforms too.

[oconnor663]

Hmm, is this some weird GitHub Actions limitation on CI changes? Lemme make a branch of my own and push this.

[oconnor663]

Ah no I see the problem. The build matrix for the new jobs doesn't have a target field, so you need to do this:

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index dec49e2..3ed564b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -236,8 +236,8 @@ jobs:

   # CMake build test (Library only), current macOS/Linux only.
   cmake_build:
-      name: CMake ${{ matrix.target.os }}
-      runs-on: ${{ matrix.target.os }}
+      name: CMake ${{ matrix.os }}
+      runs-on: ${{ matrix.os }}
       strategy:
         fail-fast: false
         matrix:

Very frustrating that Actions handles this error by silently dropping the jobs...

[SteveGremory]

made the changes, perhaps it should work now?

[oconnor663]

(I tried to resolve a merge conflict by clicking a button here on GitHub, and I'm really not sure I did it right...)

[SteveGremory]

Figured it out, I had to send it for a re-review (makes sense) so if @oconnor663 approves of it now, it will probably be merge-able. Sorry that I kind of deserted the PR after a while 😔

[BurningEnlightenment]

I would like to propose a few changes (less flag hardcoding, symbol visibility macros, target_sources() instead of accumulating sources in a macro, windows/msvc compat) to the CMake buildsystem. But before I invest time I'd like to know, whether the blake3 team is generally interested in adding CMake build files.

[oconnor663]

I've rebased this branch and manually landed it on master. Still don't understand how reviews work but now I don't have to :) Thanks @SteveGremory!

@BurningEnlightenment I'd love more PRs to improve things. I'm not sure what the modern CMake idioms are. Also Windows compat sounds great if you have the interest.