The glib-genmarshal tool from GLib 2.54 added various command line
arguments that allow us to remove a bunch of as hoc manipulations of
the generated marshaller source files. The marshal generator tool can
now include an header in the source, and undef the G_ENABLE_DEBUG
pre-processor symbol for us. It can also generate the prototypes of the
marshallers in the C source, and avoid a 'missing-prototypes' compiler
warning.
