> I am not use to pkg-config.
> Most the time I cut past in my Makefiles the compile command lines given
> in the docs. But I haven't find it in the Gegl doc.
pkg-config has over the last years become the de-facto standard to
obtain compiler and linker flags for a library. I don't think we need
to go into the details here. You probably want to read the pkg-config
man-page and this chapter from the GTK+ docs: