Bug#920824: libunistring: please disable --as-needed

[Bruno Haible]

> Maybe eventually Debian will use --as-needed by default too

In this case, I would suggest to apply the fix that has been done
upstream:
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=cca32830b57e91f837c01d15b8732f23ff97fc36

Bruno