configure.ac: updates to quiet obsolete macro warnings

Clears autoconf warnings:

configure.ac:38: warning: The macro 'AC_PROG_LIBTOOL' is obsolete.
configure.ac:38: You should run autoupdate.
aclocal.m4:3465: AC_PROG_LIBTOOL is expanded from...
configure.ac:38: the top level
configure.ac:51: warning: The macro 'AC_HELP_STRING' is obsolete.
configure.ac:51: You should run autoupdate.
./lib/autoconf/general.m4:204: AC_HELP_STRING is expanded from...
configure.ac:51: the top level