RFR: JDK-8249292: DependOnVariable macro fails on empty value

[Erik Joelsson]

The DependOnVariable macro fails if the variable is empty. The failure 
is rather non intuitive as it complains that the vardeps file doesn't 
exist and is needed for the target. This patch make the macro treat an 
empty variable value just like any other value. I'm also adding test 
cases to verify this new behavior.

Bug: https://bugs.openjdk.java.net/browse/JDK-8249292
Webrev: http://cr.openjdk.java.net/~erikj/8249292/webrev.01/index.html

/Erik