debug/gdb: fix $need_ncurses_src usage

It should be used only to decide whether we need to download/extract
ncurses, not wheter we should build it or not.

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"