do not print config.log if there were no errors

[jeffhammond]

it is bloody impossible to find the actual error in Travis logs,
particularly the raw output, when config.log is present, because it
contains a large number of innocuous instances of the word "error"
associated with configure tests that always fail.

question for reviewers: do people want to have config.log even when the Travis failures occur in later steps? maybe some people do.

[evaleev]

the failing logs show this:

travis_time:end:1e4d6f28:start=1492102572592879603,finish=1492102574170422811,duration=1577543208
�[0Ktravis_fold:end:cache.2
�[0Ktravis_fold:start:after_failure
�[0Ktravis_time:start:0274a75a
�[0K$ {:"find . -name config.log -exec grep -L "configure"=>"exit 0" {} ";" | xargs cat"}
/home/travis/.travis/job_stages: line 53: {:find . -name config.log -exec grep -L "configure=: command not found

[jeffhammond]

Yeah, I've been seeing that error for reasons I do not understand. That shell command definitely works locally. I'll debug.