RFR (XS): JDK-8141548 Hotspot Windows build should respect WARNINGS_AS_ERRORS

Thu Nov 5 16:18:06 UTC 2015

Currently the /WX flag is hardcoded in the Hotspot windows make files. 
This makes it impossible to turn of warnings-as-errors for Windows 
without explicitely hacking the makefiles. This should be fixed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8141548
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8141548-hotspot-windows-build-should-respect-warnings-as-errors/webrev.01

/Magnus