RFR: JDK-8237879: make 4.3 breaks build

[Tim Bell]

Erik:

> Please review this fix to make the build compatible with GNU make 4.3. 
> See bug for details on the fix, but this change basically adds an empty 
> rule like this for the example failure:
> 
> /localhome/git/jdk-sandbox/open/build/linux-x86_64-server-release/buildtools/langtools_tools_classes/_the.BUILD_TOOLS_LANGTOOLS.vardeps: 
> 
> 
> With this change I'm able to build with make 4.3 and DependOnVariable is 
> still behaving as expected.
> 
> Webrev: http://cr.openjdk.java.net/~erikj/8237879/webrev.01/index.html
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8237879

Looks good.

Tim