Define APEX_LINUX if Linux in CMakeLists.txt directly

I'm using performance_test as a standalone lib with ROS 2, so APEX_LINUX isn't defined by any external/system toolchain file.

There are many other ways/places to do this in the CMakeLists.txt file, so let me know if you prefer something else.

I've added a second commit that throws an exception if a user tries to set CPU affinity on an unsupported platform (not Linux, not QNX). I think this is better than silently not doing something that a user wanted to do.

Signed-off-by: Christophe Bedard bedard.christophe@gmail.com