build: Use sed for --with-libtool-modification.

*  configure.ac (--with-libtool-modification): Use sed.

I crafted a sed script so that it can work well against multiple
versions of libtool. Tested with 2.4.2 and 2.4.7.

• GnuPG-bug-id: T6619
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>