libtool: make sure autoheader run before autoconf

autoheader will update ../libtool-2.4.6/libltdl/config-h.in which
autoconf needs, so there comes a race sometimes as below:
 | configure.ac:45: error: required file 'config-h.in' not found
 | touch '../libtool-2.4.6/libltdl/config-h.in'

So make sure autoheader run before autoconf to avoid this race.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d8451cbe)
Signed-off-by: Steve Sakoman <steve@sakoman.com>