configury: does dlsym require leading symbol name underscore?

* m4/module.c (uscore_sym): Change NEED_USCORE guarde to
LDSYM_USCORE.
* configure.ac: Move compiler characteristics checks above libtool
initialization section.
(Dynamic Loader Characteristics): Determine whether dlsym requires
a leading symbol name underscore, without requiring that self
dlopening works like LT_FUNC_DLSYM_USCORE macro does.
Reported by KO Myung-Hun

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