* 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
1. When more than 2 streams are piped, we use threads to read/write data
parallely. In this scenariod read_atmost_n API was being used instead of
read_all.
2. Another issue with check_output argument validation fixed.
