can't find X includes in dapper

[golfer]

I'm trying to compile kmodemlights from kde-apps into Dapper and the
configure fails with the error "checking for X... configure: error:
Can't find X includes. Please check your installation and add the
correct paths!"

I created Dapper from the Breezy installation being upgraded from the
Dapper repositories via apt-get dist-upgrade.  I did not have the
X-Window-System-Dev package installed before the upgrade, so it didn't
get upgraded.

Now, I can't seem to figure out the naming for the X dev packages. 
There does not appear to be one dummy package to pull it all in.  I've
installed a bunch of the ones I could find with a normal search,
including:

libx11-dev
libxau-dev
libxdmcp-dev
libxext-dev
libxi-dev
libxxf86misc-dev
x11proto-core-dev
x11proto-fonts-dev
x11proto-input-dev
x11proto-kb-dev
x11proto-render-dev
x11proto-video-dev
x11proto-xext-dev
x11proto-xf86dri-dev
x11proto-xf86misc-dev
xserver-xorg-dev
xserver-xorg-driver-fbdev
xserver-xorg-input-evdev

The kmodemlights configure script appears to be testing the presence
of the x11 includes with this configure script line:  test -z
"$x_direct_test_include" && x_direct_test_include=X11/Intrinsic.h

I do not have this 'Intrinsic.h' file on my system yet, but I also
don't find any more x11 dev packages to install.  Can someone using
Dapper check to see if it exists on your system (i.e. locate
Intrinsic.h) and let me know which packages I need to get it?

TIA