* libtool.m4: Update support for NetBSD
* ltdl.m4: Make sure that libltdl_cv_func_dlopen and
libltdl_cv_lib_dl_dlopen are set and used instead of the
ac_cv_ versions. Also, use lt_cv_sys_global_symbol_pipe
instread of global_symbol_pipe. This fixes the detection of
dlopen on BSD derived plaforms and more importantly fixes
need_uscore detection.