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.
gimpthumb-enums.c is a built source from the header. No need to scan it.
As a side effect, it fixes another build break as it could not recognize
the construction rule for this file.
Thanks to tmanni for raising these build issues.
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!