Fix libtool 2.4.2 to correctly detect .def files.

* ltmain.sh (sed_uncomment_deffile): New.
(orig_export_symbols): Uncomment def file before testing for EXPORTS.
* m4/libtool.m4: Do the same for the generated code.

The old code was not correct in that it only looked at the first line
and puts an EXPORTS keyword in front if missing. Binutils 2.22
accepted a duplicated EXPORTS keyword but at least 2.23.2 is more
stringent and bails out without this fix.

There is no need to send this upstream. Upstream's git master has a
lot of changes including a similar fix for this problems. There are
no signs that a libtool 2.4.3 will be released to fix this problem and
thus we need to stick to our copy of 2.4.2 along with this patch.

• Signed-off-by: Werner Koch <wk@gnupg.org>