Failing to include mdspan in CMake through CPM

[aosterthun]

Hi,

I've been trying to include mdspan in a project of mine. Since I use CPM to manage other dependencies I tried to do the same with mdspan. Sadly I can not get this to work as expected. When trying to include the header for mdspan I get a No such file or directory error.

I've build a small test project to reproduce the error for debugging. The project includes a Dockerfile describing the environment (it can also be used in VSCode as a devcontainer) as well as build instructions.

[masterleinad]

Works with the latest commit (but not the latest tag), probably fixed by #258.