* Support CMake 4
CMake 4 has removed compatibility with CMake < 3.5
Bumping minimum required version to 3.5 enables
CMake 4 to build this code.
* Move header into subdir
* Improve CMake setup
This commit configures and installs CMake metadata files. This also
provides the namespaced ALIAS target `cpp-subprocess::subprocess`.
* Update README with CMake instructions
* Update include paths in tests