[patch] Add gdb.tar rule to top-level Makefile.in

[Andrew Cagney]

> Done. Found two things:
> 
> - BINUTILS snapshot process relies on GNU make
> - the new do-bz2 rule was (re-)creating the tar ball created by do-tar.
> 
> Neither were affecting GDB.  I've fixed both and committed the change.

Oops, fixed the second.  The BINUTILS snapshot appears to still rely on 
GNU make.

Andrew