Add cmake support

[andreasbuhr]

Pull request https://github.com/lewissbaker/cppcoro/pull/110 by @mmha was opened 1.5 years ago, but was never finished. @dutow later based his fixes for gcc on it. In https://github.com/lewissbaker/cppcoro/pull/169, CMake support was again proposed by @Garcia6l20.

This pull request should become a polished pull request which incorporates material from all those pull requests to add CMake support to cppcoro.

Any help or review is greatly appreciated. Especially, help in testing with MSVC would be great.

To help improve this pull request, please fork https://github.com/andreasbuhr/cppcoro/tree/add_cmake_support and create a pull request toward this branch.

[andreasbuhr]

This pull request is now open for two month. I now stop maintaining this pull request. From now on, I will continue to maintain the master branch in https://github.com/andreasbuhr/cppcoro. There we have CMake support, a CI based on Github Actions, and support for the latest compilers.