gnutls: load libunistring-optional gnulib module

gnutls: load libunistring-optional gnulib module

Since a few days staging_dir/host/share/aclocal/ contains new m4 files
(libunistring-base.m4, libunistring-optional.m4, libunistring.m4 etc.)
that get applied during autoreconf. This changes the libunistring setup
enough that we run into problem (builds fail).

Load the libunistring-optional gnulib module in configure.ac to be able
to add/use the new configure switch "--with-included-libunistring".

This is at most a workaround. This should be followed up with gnutls
upstream to clean this up.

Signed-off-by: Sebastian Kemper <redacted>