Always rebuild on errors

https://groups.google.com/forum/#!msg/ninja-build/YQuGNrECI-4/ti-lAs9SPv8J
discusses a case where an rule updates its output file and then
fails.  The next run of ninja considers the ouptut file clean
and doesn't rebuild it.  Always stat output files after they are
built, and write the mtime into .ninja_log.  Consider output files
dirty if the recorded mtime is older than the most recent input
file.
7 files changed