QNX: Do not disable compiler extensions for CMake itself

The `set(CMAKE_CXX_EXTENSIONS FALSE)` option has the effect of passing
compile option `-std=c++NN` instead of `-std=gnu++NN`.  On some
platforms, the latter form (or the secondary effects that it has, such
as setting _XOPEN_SOURCE) is required.  This typically affects platforms
such as mingw, cygwin and QNX.  The GNU-like compiers default to
`-std=gnu++NN`, which means that users can typically build source code
on those platforms by default.

While the `set(CMAKE_CXX_EXTENSIONS FALSE)` option was set here in
commit f034b0f6 (CMake compilation: do not use compiler extensions,
2020-03-14), the other changes in that commit added `#defines` which
become required in the absense of use of `-std=gnu++NN`.  However, only
platforms regularly tested in the cmake dashboard were ported, as is
reasonable.  That made CMake fail to compile on QNX and perhaps other
platforms which for which no one is submitting regular testing to the
cmake dashboard.

Make the `set(CMAKE_CXX_EXTENSIONS FALSE)` option conditional on non-QNX
to restore the QNX build.

Issue: #21503