NDEBUG

[Scott Goehring <scott poverty bloomington in us>]

Can we get this changed (in gtkobject.h) to something a little less
generic?  NDEBUG is generically used to disable application debugging;
library debugging should have its own symbol.  (Someone might want to
disable library debugging without disabling application debugging.)  I
would suggest using GTK_NDEBUG, or, alternatively, using GTK_DEBUGGING
or GTK_DEBUG_CASTS to selectively enable cast debugging.  The latter
options especially make more sense in a production environment, which
is what we're heading for with 1.0 anyway.