autogen.sh sets DATE_WHEN_GENERATED to empty string on Solaris version 11 or later

This changeset wiped out DATE_WHEN_GENERATED:

http://hg.openjdk.java.net/build-infra/jdk8/rev/01e8883b0252

Fails later when trying to configure and build the source tree on systems that do not have autoconf, because the files look out of date:

  Warning: The generated configure file contains changes not present in the custom generated file.
  Cannot locate autoconf, unable to correct situation.
  Please install autoconf and run 'bash autogen.sh' to update the generated files.
  Error: Cannot continue