Having recently switched to fluxbox (nice!) I'm trying to compile a couple of dockapps. Unfortunately they all barf at the configure stage with the message:

checking for XShapeCombineMask in -lXext... no
The shape extension stuff could not be found in the X client libraries

I have installed the debian packages: libxext (4.3.0-7 -- "supported protocol extensions: ... SHAPE, the non-rectangular shaped window extension") and libxext-dev (4.3.0-7), and am at a loss as to why this is happening. Can someone direct me to the appropriate library to install?

Thanks