RFR: JDK-8141696 Improve COMPARE_BUILD

Mon Nov 9 10:09:37 UTC 2015

The new COMPARE_BUILD feature has proven valuable to build system 
development. However, after use it leaves the build system in a 
different state than prior to use, which requires manual intervention 
for multiple runs.

This patch fixes so that the build system is restored to the starting 
point (except for unhandled crashes, e.g. in configure). It also moves 
some error handling logic to InitSupport.gmk so it does not clutter 
Init.gmk.

Bug: https://bugs.openjdk.java.net/browse/JDK-8141696
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8141696-improve-COMPARE_BUILD/webrev.01

/Magnus