Support for response files with $NM.

Support for response files with $NM.

* libltdl/m4/libtool.m4 (_LT_CMD_GLOBAL_SYMBOLS)
<nm_file_list_spec>: New tag variable. Set it to '@' if input
files can be passed to $NM in a file named with the '@' option.
* libltdl/config/ltmain.m4sh (func_mode_link): When
nm_file_list_spec is nonempty, use it to avoid skipped_export.
* doc/libtool.texi (libtool script contents): Document
new variable.