Fix assorted compiler warnings on Mac OS X.

BUILD/SETUP.sh:
  Remove -Wctor-dtor-privacy flag to workaround a GCC bug that
  causes it to not properly detect that implicitly generated
  constructors are always public.
cmd-line-utils/readline/terminal.c:
  tgetnum and tgetflag might not take a const string argument.
mysys/my_gethostbyname.c:
  Tag unused arguments.
mysys/my_sync.c:
  Tag unused arguments.