examples: stop processing meson file if build impossible

examples: stop processing meson file if build impossible

Once it has been determined that an example cannot be built, there is
little point in continuing to process the meson.build file for that
example, so we can use subdir_done() to return to the calling file.
This can potentially prevent problems where later statement in the file
may cause an error on systems where the app cannot be built, e.g. on
Windows or FreeBSD.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>