Imported from glulxe commit c1c3a31403916e8262e27b6e1a6fcda2085941f3.
Added glk.h,gi_blorb.h,gi_dispa.h from https://raw.githubusercontent.com/erkyrath/cheapglk/master/.
Added gtblorb.c from https://raw.githubusercontent.com/erkyrath/glkterm/master/gtblorb.c.