Fix for segfaults induced by the following script:

button .b
pack .b
update
destroy .b
button .c
update
dgp's constant comparison fix
An improvement on before
Fix from kevin_walzer for buttons in cocoa
Apply a similar dealloc fix to menubutton
Stop segfaults on Mac OSX after [$button destroy].
Stop segfaults on Mac OSX after [$button destroy].
autoconf
merge trunk