How to fix autoconf warning: The macro `AC_PROG_LIBTOOL' is obsolete.

Problem

When running autoconf, you see an error message such as

configure.ac:29: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
configure.ac:29: You should run autoupdate.
aclocal.m4:122: AC_PROG_LIBTOOL is expanded from...
configure.ac:29: the top level

Solution

Before running autoconf, you need to run autoupdate to work around this issue.

autoupdate

After that, you can run autoconf again and the error will be gone.