This will be the build system going forward, so it seems sensible to
support it right away. There's no neat way to use the system
GameNetworkingSockets upstream yet, so we add a hack for now.
Proactively decrease platforms to Linux only since the CMake doesn't
quite use GNUInstallDirs on Darwin and Windows unfortunately.
pname is still soldat, regardless of whether it is stable or unstable,
attribute name should keep the suffix for clarity (soldat stable is
unfree (?) and not packaged by us atm).
