[RFA/gdb-9-branch] Abort configure immediately if building GDB in tree

[Eli Zaretskii]

> From: Sergio Durigan Junior <sergiodj@redhat.com>
> Cc: brobecker@adacore.com,  simark@simark.ca,  gdb-patches@sourceware.org,  tom@tromey.com
> Date: Fri, 31 Jan 2020 16:39:49 -0500
> 
> OK, I think I have something that works.  When 'configure' is invoked
> with '--enable-src-release-build', a tarball is generated and, as far as
> I have checked, contains the correct snapshot.
> 
> WDYT?

It certainly looks like something I had in mind, but since I know very
little about sourceware configury, I hope others will chime in.

Thanks.