debug/gdb: install dependable libs in a generic target static libs dir

For now, ncurses is the only dependable target library built for gdb.
But expat is coming, and there's no reason to install each library in
its own place.

So, install ncurses in a generic directory, where other dependable
libraries can be installed as well.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 COPYING