/usr/lib/grub/grub-mkconfig_lib is in grub-common 2.02~beta2-22+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/grub/../../share/grub/grub-mkconfig_lib
/usr/lib/grub/../../share/grub/grub-mkconfig_lib