Broken Cmake scripts

[ChuanqiXu9]

Search before asking

• [x] I searched the issues and found no similar issues.

What happened + What you expected to happen

Extract from https://www.reddit.com/r/cpp/comments/ta4h29/async_simple_a_c20_coroutine_library/

- C++20 support was added in 3.12, this project wrongly requires 3.8
- Pulling in dependencies on the main code path for developer requirements (tests, benchmarks)
- Setting user-settable variables (CMAKE_BUILD_TYPE which is also completely ignored by multi-config generators)
- Hardcoding flags (put them in a toolchain/preset)
- Lack of install interface

Are you willing to submit a PR?

• [ ] Yes I am willing to submit a PR!