This includes atk/atkversion.h in all the public headers, either directly
or via atk/atkobject.h, and annotates the public symbols in the headers,
which all lead to _ATK_EXTERN via one of ATK_AVAILABLE_IN_ALL,
ATK_AVAILABLE_IN_X_Y, ATK_DEPRECATED, ATK_DEPRECATED_FOR,
ATK_DEPRECATED_IN_X_Y or ATK_DEPRECATED_IN_X_Y_FOR, depending on which
stable release series the API was introduced or deprecated.

_ATK_EXTERN which can then be defined in a way during the build, so that
these symbols can be exported automatically using compiler directives.

Also use _ATK_EXTERN for ATK_VAR in atk/atkmisc.h during the build so that
variables can also be properly exported.

https://bugzilla.gnome.org/show_bug.cgi?id=728031