modules: support _ symbol prefix requirement for dlsym. * configure.ac (LT_FUNC_DLSYM_USCORE): Call it to set NEED_USCORE. * m4/module.c (uscore_sym): New function. Use this to prepend an underscore if NEED_USCORE was set at configure time. Reported by KO Myung-Hun Signed-off-by: Gary V. Vaughan <gary@gnu.org>