Properly configure Coroutines cmake module

[karzhenkov]

There are some problems when using cppcoro from an external project:

• the installed library cannot be used if the final version of C++20 coroutines is not supported by toolset;
• the library cannot be used directly from its build tree, which would sometimes be convenient.

The first commit adds tests that attempt to build an executable using cppcoro:

• app_build_with_cppcoro_install_dir
• app_build_with_cppcoro_build_tree
• helper tests to install cppcoro and configure the application build tree.

Following commits make the tests successful.

This PR should probably go to add_cmake_support, but that branch looks somewhat outdated.

[andreasbuhr]

Hi @karzhenkov ,
thanks a lot for your feedback, I'll have a look tomorrow.
I do not maintain the add_cmake_support branch any more, see my latest comment in lewissbaker#174 .

[bhundt]

Any update of integrating this into the main branch? I stumbled upon the same problem and it would be nice to be able to just clone the main branch from my build system. Thank you guys for your efforts!