RFR: JDK-8149479: Fix compare.sh to have a clean baseline with COMPARE_BUILD

[Erik Joelsson]

Hello,

Please review these fixes for compare.sh and the COMPARE_BUILD flag for 
make.

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. 
Which I now have.

I also changed COMPARE_BUILD to fail the build when differences are 
found and added an option to COMPARE_BUILD to disable failing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8149479
Webrev: http://cr.openjdk.java.net/~erikj/8149479/webrev.01/

/Erik