The situation is this: even when we --disable-gtk-doc, if gtk-doc is
actually installed at configure stage, tools like gtkdoc-rebase will be
picked up and run during the install target. That is bad because the
gtk-doc package may have been removed by then, especially during dpb(1)
bulks (we explicitely disable support for it so why should it stay...).
So for now, let's add the following env to configure whenever we use
--disable-gtk-doc, until a bettersolution is found...
CONFIGURE_ENV +=ac_cv_path_GTKDOC_CHECK="" \
ac_cv_path_GTKDOC_REBASE="" \
ac_cv_path_GTKDOC_MKPDF=""
