Ignore 'compile errors' that occur during successful configures

[karkhaz]

At the moment, we flag up anything in the log that looks like an error. This includes any errors that occur during configure, even if the entire configure invocation exited with 0 (indicating that those 'errors' were actually expected).

We should not attempt to identify errors in config logs if configure exited with 0.

This is so that our results page doesn't contain false positives from configure scripts; all errors should be real errors that killed the build process. This issue depends on #99.