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 leave behind gimpcolorscale, gimpcolorscales and gimpcolorselect
for now. There are some GI compilation errors and I don't have the time
right now to look into it. Most of libgimpwidgets is available to GI
bindings now anyway.