A build output of such a failure can be found here:
http://hydra.nixos.org/build/21994789/nixlog/1/raw
The build_ext command doesn't return non-zero if it fails but instead
produces a file with a "_failed.so" suffix, which we're now checking
after the build_ext command.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>