The new --body and --header args are broken because they do not allow
the use of --output-directory to set the correct `#include "foo.h"`
line in `foo.c`. The changes in the gdbus test case show this.
Disabled till this can be fixed in glib.
Closes https://github.com/mesonbuild/meson/issues/3488
Add new 'docbook' argument which generates Docbook documentation for
each D-Bus interface. The docbook argument will be used as prefix
in `PREFIX`-NAME.xml pattern, and NAME will be replaced by the D-Bus
interfaces.