I've moved the non-recursive-gnulib-prefix-hack module to
gnulib, with two small improvements, so remove it from here
and update gnulib to the latest.
* gl/build-aux/prefix-gnulib-mk: Remove file.
* gl/m4/non-recursive-gnulib-prefix-hack.m4: Remove file.
* gl/modules/non-recursive-gnulib-prefix-hack: Remove file.
* gnulib: Update to latest.