RFR: JDK-8035495 Improvements in autoconf integration

[Tim Bell]

Hi Magnus:

> This bug covers several small improvements in how our configure 
> eco-system integrates with autoconf. More specifically:
>
> * Fix so config.status --recheck works.
> * Let the top-level configure script determine TOPDIR, and pass it on.
> * Clean up usage regarding TOPDIR, SRC_ROOT and AUTOCONF_DIR.
> * Prohibit direct execution of common/autoconf/configure; always use 
> configure in top-level dir.
> * Add "do not edit" header to generated-configure.sh
> * Add output on how we determine where to store the configuration.
> * Improve output from autogen.sh, including autoconf version.
> * Unify duplicated code in autogen.sh
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8035495
> WebRev: 
> http://cr.openjdk.java.net/~ihse/JDK-8035495-improvements-in-autoconf-integration/webrev.02

Looks good to me.  Thanks for adding the 'do not edit' banners.

Tim