[10/10] RFC: remove gdb_wait.h

[Eli Zaretskii]

> From: Tom Tromey <tromey@redhat.com>
> Date: Thu, 15 Nov 2012 12:34:09 -0700
> 
> This replaces gdb_wait.h with the corresponding gnulib module.

This could break the MinGW build some day, or maybe even today: its
definition of WIFSIGNALED and WIFEXITED is inaccurate for Windows.

I say "some day" because it looks like we don't actually use
WIFSIGNALED in any file that is compiled on Windows (not sure about
WIFEXITED, though).