Is there a special reason why you are compiling fluxbox from source code as opposed to just using the Debian package tools to install a compiled package? You shuold probably use the Debian package management system. First check to see if fluxbox is a supported Debian package. Visit http://packages.debian.org, plug in fluxbox and set the engine to search "any" distribution & "any" section. Then we get the results and find Debian has the latest version of fluxbox in both testing & unstable.

Next update your system's package lists with apt-get update. Then install fluxbox with apt-get install fluxbox or you can use a GUI tools like synaptic.

I hope this helps.
James