build: Update libtool from version 2.4.7.

* build-aux/compile, build-aux/depcomp, build-aux/ltmain.sh: Update.
* m4/libtool.m4, m4/ltoptions.m4, m4/ltsugar.m4: Update.
* m4/ltversion.m4, m4/lt~obsolete.m4: Update.

This is a breaking change because we have been maintaining local
changes. It is going to be fixed by the following commit.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>