The rather new feature COMPARE_BUILD, which builds twice, applying some kind of change between them, is really neat. Especially when run through JPRT to check all platforms at once. The problem is that compare.sh currently isn't clean on all these platforms. There are also some special peculiarities particular to when JPRT is running the build which confuses compare.sh.

Given that COMPARE_BUILD uses the exact same output directory, and JPRT sets the version-opt string to a fix value for both builds, we should be able to achieve a clean and stable baseline for an empty patch file.

I would also like to change COMPARE_BUILD to fail the build when differences are found. I will add another option to COMPARE_BUILD to disable failing.