qnx 7.0 support
Added conditionals in meson.build to distinguish general qnx from qnx 7.0 from qnx 7.1. Added implementation for libc stuff missing in qnx 7.0 such as openat(...) and fdopendir(...)
This implementation requires setting host_system to 'qnx-x.x' rather than just 'qnx' in the croo file to let the build system recognise the version
Edited by Klaus Holst Jacobsen