As I don't want to type all that stuff again, about why, and how I did - let me give you the short form - gstreamer needs glib. It was installed in my scrachbox, but it did not have its include path setup in the build.
The solution was to add the following, the the INCLUDES section of the makefile that also makes the gstreamer decoder:
I'm not sure if it is the correct place to do it, nor if it is done in the right way, but it makes sence that it is working if you do it in this place - I always get a little worried when I see stuff like ../../../ in makefiles, as it can lead to "funny" issues if you at another time want to move anything around in the tree.
(The gstreamer line was added previously to allow inclusion of the gstreamer header files)
The ifdef MOZ_GSTREAMER protection, is also something that should be considered agian, while it's probably fine for the gstreamer directory, putting the "generic" glib includes under the same protection could lead to problems if any other code startet to rely on glib too. - It will work find as long as gstreamer is included in the build, but if it isn't then the header files for glib cant be found. If another entity includes them too, then we might have multiple entries pointing the same include directory - not good either, as a messed up include dir can increase compile times significantly - for a project this size I don't know if it will matter thou...
1 down x to go - Next entry about not found gst/* files :) - I now cross my fingers that this entry will be accepted by the server.... (Ctrl+A Ctrl+C)