Following Vladimir's suggestion on IRC, the attached patch fixes
bug http://savannah.gnu.org/bugs/?35452.

Grégoire

p.s. The patch uses @PACKAGE_TARNAME@ instead of @PACKAGE@ in the
name of the directory that grub-mkconfig_lib is sourced from, to
preserve the definition of pkgdatadir that was in grub-mkconfig.in.
But I guess that those two are always equal for GRUB.