The usual way to define a macro in is to name macro with 'USE_xxx' when
they are used to enable a feature 'xxx'.
As the feature concerns the Dock and not DnD in general (WINGs support is
not disabled), make it a bit more clear in the macro name and document the
configure flag in the Installation Manual.
Took opportunity to compile the corresponding file only when the feature is
not disabled.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
b4cf94aacd