debug/gdb: fix the mess do_{gdb,expat,ncurses} have become

Rename those three variables to properly reflect their purpose: decide
whether we need to download/extract gdb/libexpat/libncurses, not whether
we need to build them or not.

This is only a rename for now, subsequent changes will further
fix this mess.

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"