ncurses 5.6 - patch 20080329

ncurses 5.6 - patch 20080329

+ add null-pointer checks in set_term() and delscreen().
+ move _nc_windows into _nc_globals, since windows can be pads, which
  are not associated with a particular screen.
+ change use_screen() to pass the SCREEN* parameter rather than
  stdscr to the callback function.
+ force libtool to use tag for 'CC' in case it does not detect this,
  e.g., on aix when using CC=powerpc-ibm-aix5.3.0.0-gcc
  (report/patch by Michael Haubenwallner).
+ override OBJEXT to "lo" when building with libtool, to work on
  platforms such as AIX where libtool supports only static libraries
  (report/patch by Michael Haubenwallner).
+ add configure --with-pthread option, for building with the POSIX
  thread library.