* 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
While this is header-only, and users are expected
to download the header and commit it to their project, it's better to
have unique namespace for the header by default.
This has two reasons:
1. I wish to enable installing it with conan.io, and want to reduce
the possibility for namespace clashes. The term "subprocess" is
pretty overloaded.
2. Currently the README and the CMakeLists differ in their include
path, it's better to be consistent, even though users can put it
wherever they please in their project.
Since most users are using it as header only anyway, I don't think it'd
make much diference.
Co-authored-by: Elazar Leibovich <elazar.leibovich@nextsilicon.com>
