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

[Eli Zaretskii]

> From: Sergio Durigan Junior <sergiodj@redhat.com>
> Cc: Simon Marchi <simark@simark.ca>,  gdb-patches@sourceware.org,  Tom Tromey <tom@tromey.com>
> Date: Thu, 30 Jan 2020 16:06:18 -0500
> 
>   $ ./src-release.sh -x gdb
>   ==> Cleaning sources.
>   ==> Making gdb-9.0.90.20200130/
>   ==> configure --target=i386-pc-linux-gnu  --disable-binutils --disable-gas --disable-gold --disable-gprof --disable-ld  --enable-gdb --enable-libctf --enable-libdecnumber --enable-readline --enable-sim
>   configure: error: GDB must be configured and built in a directory separate from its sources.
>   ...
> 
> I'm wondering what to do here.  I think I found a hacky and ugly
> solution here, but I'm trying to see if there's a better approach.

A special command-line switch to 'configure', to be used by
src-release.sh?