This extension allows to interact with processes through PTY, using expect library.
You probably shouldn't be building from these sources. Instead, you should be installing the package via apt-get:
sudo apt-get install libexpect-php5
If you were looking to build from sources (for some god awful reason, who wants to be a Debian package maintainer anyway?), you should probably be using the tools in devscripts. Read up on the Debian New Maintainers' Guide for details on package maintenance.