build: new module to convert lib/ to non-recursive make

build: new module to convert lib/ to non-recursive make

* gl/modules/non-recursive-gnulib-prefix-hack: New module.
* gl/m4/non-recursive-gnulib-prefix-hack.m4:
(gl_NON_RECURSIVE_GNULIB_PREFIX_HACK): This is the snippet
that this module inserts near the end of configure.
* gl/build-aux/prefix-gnulib-mk: New script, from bison.
Changes from the code in bison:
(prefix_assignment): Split a long line.
(prefix): Add trailing slashes to avoid a single false match.
Prefix imaxtostr.c and the other *tostr.c file names manually.
Also, use $prefix in place of hard-coded "lib/".