New libtoolize --no-warn option and LIBTOOLIZE_OPTIONS parsing.

New libtoolize --no-warn option and LIBTOOLIZE_OPTIONS parsing.

* libltdl/config/general.m4sh (opt_warning): New option.
(func_warning): Only display a warning message if $opt_warning
is true.
* libtoolize.m4sh: Accept options in LIBTOOLIZE_OPTIONS
environment variable.
(--no-warn): Parse new option.
(func_serial_update_check, func_keyword_update): Use func_warning.
(func_check_macros): Bail out before style issues nags when
$opt_warning is false.
* doc/libtool.texi (Invoking Libtoolize): Document it.
* tests/libtoolize.at: Test it.
* NEWS: Updated.