Add Main-Class for cpp-build tools JAR.

Add Main-Class for cpp-build tools JAR.

- Adds the manifest entries to the basic tools JAR.

- Creates a symlink named like the dependencies JAR that was used
  previously.

- Creates a symlink to the protobuf JAR.

In the manifest, the classpath reference to the protobuf JAR is a
relative filename. The second symbolic link satisfies that by linking to
the installed JAR.

This is only needed for people building packages

People who build normally with Maven will find that "mvn package" still
produces the original tools JAR which has its own Main-Class attribute

Author: daniel@pocock.pro