Not possible to disable statx syscalls

Even if one passes -no-feature-statx, the code in https://github.com/qt/qtbase/blob/5.10/src/corelib/io/qfilesystemengine_unix.cpp#L98 makes Qt use the statx syscall whenever the kernel of the system it's being built on supports it. This causes serious issues: for instance, in systemd 235 there is a whitelist for allowed syscalls in containers and syscalls not known by libseccomp (such as statx) are blocked. This makes it impossible to build Qt (or most Qt applications) in a systemd container with systemd 235 and kernel >=4.11.

 This can also potentially cause issues when trying to run Qt libraries compiled on a system with kernel >=4.11 on another system with an older kernel (such as the current LTS series)