[bug-libunistring] gnulib integration

[Pádraig Brady]

I've started integrating libunistring functionality into
coreutils, which already uses gnulib.

I was unsure though of the cross-over between
gnulib and libunistring. When I was first testing
it I noticed that once I used gl_LIBUNISTRING
then I needed to link printf with libunistring to
get u8_uctomb_aux, and I noticed that unistr.h
was not generated by gnulib, and instead the
libunistring version was used.

In recent testing (probably since support for
libunistring-optional was added?) unistr.h is
now generated by gnulib even in the case when
using libunistring.
Now I have the additional "issue" that I must
include the unistr/u8-mbtouc-unsafe module
if I want to use that function in my code.
The existing unicodeio module requiring linking
with libunistring for u8_uctomb_aux issue still exists.

So in summary, is the required inclusion of the
unistr/u8-mbtouc-unsafe module expected,
and is there a way to get unicodeio to use
the gnulib version of u8_uctomb_aux?

cheers,
Pádraig.