RFR: JDK-8072106 Properly handle dependencies for deleted header files

[Magnus Ihse Bursie]

When a header file is deleted, make will complain "No rule to make 
target <old header file>". This often breaks incremental build 
completely unnecessary.

This can be fixed by adding dummy rules for all header files like this:
<header file>:

gcc can create such rules by itself by using -MP in the generated make 
dependency files, but that feature is not available for any other 
compiler, so I opted not to use it, in favour of a solution that works 
on all platforms.

This patch also contain a cosmetic fix for printing incremental build 
information that was made somewhat less clear when the vardeps 
functionality was introduced.

Bug: https://bugs.openjdk.java.net/browse/JDK-8072106
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8072106-make-dependency-targets-generation/webrev.01

/Magnus