RFR: JDK-8069064 Various improvements and fixes in build system

[Magnus Ihse Bursie]

 This fix is the result of preparatory work in the build-infra project. 
It includes:
* Remove duplicate detection of comm on Windows
* compare.sh enhancements and bug fixes
* Do not fail in SetupFoo macros on empty arguments
* Minor JavaCompilation enhancements
* Makefile warns for unknown control variables
* Improved "make help"
Bug: https://bugs.openjdk.java.net/browse/JDK-8069064
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8069064-fixes-from-build-infra/webrev.01

/Magnus