RFR: JDK-8168037 using ZIP as a make variable conflicts with zip's use of ZIP as an environment variable

[Magnus Ihse Bursie]

Following the standard by other tools, we set ZIP to the path of the zip 
binary.

However, ZIP is interpreted as a way to control options for the Info-Zip 
zip binary.

The solution is to rename the variable ZIP to ZIPEXE.

Bug: https://bugs.openjdk.java.net/browse/JDK-8168037
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8168037-rename-ZIP-to-ZIPEXE/webrev.01

/Magnus