I think I experienced the same phenomenon (getting stuck) once before. Later it disappeared perhaps after "make distclean" or something.
I don't know why. You may try again after removing all files in /home/slabbe/GitBox/sage/upstream/ -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-devel/c3c71686-c2e5-41b8-8434-5c04fe6e7bf1n%40googlegroups.com.