Setting IOX_NO_DISCARD in QNX

[elfenpiff]

Brief feature description

At the moment the IOX_NO_DISCARD macro is only set when the compiler is either gcc>=5 or clang>=4 or the operating system is windows. In QNX it is a NOOP. Since the qcc compiler should also support [[nodiscard]] and [[__attribute__((warn_unused))]] we should test it and activate it on QNX.