Mirroring internal request:

The config.log file gives a wealth of information about exactly what configuration was used to build a package (that uses a configure script). This information is currently missing from brew logs after a package build requiring costly action from a user (rebuild in a VM), or custom changes of the spec file to cat the output of the file to capture the content in the build log.

I think it would be valuable to save this file by default when it is produced. The chroot_scan plugin can be easily configured to search for such file and save it in a way that will not impact builds that do not produce such a file.