Factor looks for libraries named libgtkglext-x11-1.0.so (and for gdk) but on my system, the library has at least a .0 suffix, so I had to

ln -s libgtkglext-x11-1.0.so.0 /lib/libgtkglext-x11-1.0.so
ln -s libgdkglext-x11-1.0.so.0 /lib/libgdkglext-x11-1.0.so

but the correct library paths are in the generated basis/gir/*.gir files, so this step shouldn't be necessary.