configury: detect dlsym underscore prefix without dlopen self.

* m4/ltdl.m4 (LT_FUNC_DLSYM_USCORE): Compile, load and get the
address of a symbol from a separate loadable module, rather than
assuming dlopen self works.
* NEWS: Update.
Reported by KO Myung-Hun

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
gnulib: sync with upstream.

* gnulib: sync with upstream.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>