Thank you, Juanito
I had to alter the command slightly:
$ pkg-config --cflags gdk-pixbuf-2.0
and it told me that shared-mime-info was missing! I would never have guessed.
This made the GDK-Pixbuf-related warning go away:
$ tce-load -wi shared-mime-info
The OpenGL warning persisted, so I decided to not mess around and loaded every extension I could find that seemed related or that had "gl" in the name:
$ tce-load -wi libGL-dev libGLESv2-dev libEGL-dev glu-dev glade-dev gegl-dev
It was overkill, for sure, but that made the OpenGL warning go away
Solved!