Portability fixes for AIX 4.x

include/my_global.h:
  Don't define _AIX32_CURSES; this breaks AIX 4.3 (maybe others?).  If
  it is needed for some AIX systems which we support, we will need to
  define it conditionally.
ltconfig:
  Under AIX, the -bexpall flag exports all symbols except those which
  start with an underscore.  MySQL needs to export symbols which start
  with an underscore (e.g., _my_b_write), so -bexpall doesn't work.  It
  seems that -qmkshrobj does what we want.