all: Remove unused function argument

The result isn't depending on the terminal anymore, so remove that arg.
widget: Remove unused struct
a11y: Remove unused include
a11y: Fix compiler warnings about unused variables
a11y: Remove unnecessary vfunc impl

atk has a default implementation that's identical.
a11y: Remove unnecessary vfunc impl

These are deprecated in atk.
a11y: Remove unnecessary vfunc impl

Layer is WIDGET by default, so there is no need to provide an overriding impl
returning the same.
a11y: Remove unnecessary vfunc impl

Z-order is MININT by default, so there is no need to provide an overriding impl
returning the same.
a11y: Remove bogus code

There is no weak pointer there, so just remove this code.
build: Use AC_USE_SYSTEM_EXTENSIONS

... instead of some grep magic.
build: Update warning flags check
build: Remove obsolete checks
build: Default to enable bindings