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>