This provides a better error message when building with CMake and
forgetting ZEPHYR_BASE or not registering Zephyr in the CMake package
registry. See parent commit for more details (split from parent for
better readability).
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
Roughly based on eventfd example code from Linux manpages. Similarly
to other POSIX-compatible sample, Makefile.posix is provided to build
the code on a POSIX system (in this case, as eventfd() is
Linux-specific, this has to be a Linux system).
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>