Linux: use /etc/os-release to identify distros
The distro-specific version files like /etc/debian_version
and /etc/arch-release
have been generalized as /etc/os-release
as documented by the os-release(5) man page.
CMake uses these in a few places. We should consider updating the checks, though we do need to consider support for distro versions that pre-date use of /etc/os-release
.