build: Fix regenerating libxfce4ui.h after configure options change
Add a dependency on config.h
to ensure that libxfce4ui.h
is
regenerate whenever ./configure
options change. Otherwise, once
generated the header is never invalidated, and the X11 variant could
be installed with --disable-x11
or the other way around.
This is particularly a problem because X11-enabled header is included
in the source distribution, effectively preventing it from being updated
even if the package is initially configured with --disable-x11
.
Originally reported as https://bugs.gentoo.org/924164.