Need to record compile flags in log file

[mikekryjak]

I just spent quite a long time going down a rabbit hole chasing a substantial performance decrease, only to realise my compile flag has been set to Debug for the last month. The reason I missed this was because I had never changed compile flags in the past. I had done this as a quick check to debug PETSc issues while doing 5 other things at the same time and forgot about it! I didn't check because I thought I had never changed the options...

If the compile time flags were recorded in the logs, it would have enabled me to find the problem sooner. I think we should do this to prevent other users from facing the same issue - especially since compile flags are an advanced topic and most users wouldn't change or check them.

Thanks to @dschwoerer and David Dickinson for the help.

Unfortunately I can't help with this PR because I don't know how to do it.