libgimp is anyway processed at the very end after all other libgimp*
were built. This way, it also fixes#3746, by removing the $(top_srcdir)
everywhere from introspected files, hence making the build work again
with older automake.
Only libgimpwidgets is not introspected yet as it didn't work from
scratch and I don't have the time right now to look into it. Anyway with
all the others, we already have so many warnings during the GObject
Introspection step now that we have a lot of work to do already!
Michael Natterer