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>

 python

 import sys

 sys.path.insert(0, '@@PREFIX@@/share/gcc-@@VERSION@@/python')

 from libstdcxx.v6.printers import register_libstdcxx_printers

 register_libstdcxx_printers (None)

 end