No external libraries are needed to build this extension.
Optionally you can use shared memory allocation (mm), developed by Ralf S. Engelschall, for session storage. You have to download » mm and install it. This option is not available for Windows platforms. Note that the session storage module for mm does not guarantee that concurrent accesses to the same session are properly locked. It might be more appropriate to use a shared memory based filesystem (such as tmpfs on Solaris/Linux, or /dev/md on BSD) to store sessions in files, because they are properly locked. Session data is stored in memory thus web server restart deletes it.