You can run this command from anywhere in terminal.
I ran it from /tmp and / (I've done it twice).
curl http://ftp.gnu.org/gnu/autoconf/autoconf-latest.tar.gz > autoconf.tar.gz | tar -zx
cd autoconf-2.69
./configure
sudo make && sudo make install
This next step puts autoconf where it needs to be
export PHP_AUTOCONF=/usr/local/bin/autoconf