Please check your autoconf installation and the $PHP_AUTOCONFenvironment variable is set correctly and then rerun this script.

This is a very simple fix if you are running Redhat Enterprise / CentOS 5.2.  

yum install autoconf

I came across this while setting up memcache / memcached.  I had tried running phpize from the memcach-2.2.4 directory but was getting the error "Please check your autoconf installation and the $PHP_AUTOCONFenvironment variable is set correctly and then rerun this script."

Once I had autoconf installed I was able to finish the install process

./configure

make

make install

I know there are easier ways to do all of this on CentOS, but we are using a newer version of PHP (5.2.6) than came with CentOS (5.1.6), so using yum to install this would not have worked.