RFR: JDK-8237879: make 4.3 breaks build

Mon Jan 27 23:13:57 UTC 2020

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

/Erik