GTK Resource Compilation.
There's a utility called 'glib-compile-resources' that takes an xml file that is basically a list of resources such as glade ui files, and it outputs c code to embed the data in the application. It's messy to use, but it works.
There may be another messy gtk/glib utility to create these xml files that 'glib-compile-resources' can use, but if there is here's another one that may or may not be easier to use.
file: src/gtk.mkres.cpp...