[vtkusers] OPENGL_INCLUDE_DIR error message

[Levine, Zachary]

I'm trying to install VTK on a RedHat enterprise Linux 5 system.  I have the binaries for cmake.2.6.2 installed.  The next step is to configure the system in the directory VTK-bin.  So I run 

ccmake ../VTK

like it says on page 14 of The VTK User's Guide.

I get an error 

CMake Error: The following variables are used in this project, but they are
set to
 NOTFOUND.
 Please set them or make sure they are set and tested correctly in the CMake
files:
 OPENGL_INCLUDE_DIR (ADVANCED)


So, the first question is: do I have openGL?

Apparently so, because I found files: (some are links)

/usr/lib/libGLU.so.1.3.060501  /usr/lib/libGLU.so.1  
/usr/lib/libGL.so.1            /usr/lib/libGL.so.1.2

I added the following links:

/usr/lib/libGL.so
/usr/lib/libGLU.so

to help ccmake understand where the openGL libraries were.

But the variable asks about the include directory.  I'm not sure what I'm looking for, but I don't seem to have the include files.  For example, if I run the command

locate opengl

I get the following list of files

/usr/share/applications/kde/opengl.desktop
/usr/share/doc/HTML/en/kinfocenter/opengl
/usr/share/doc/HTML/en/kinfocenter/opengl/common
/usr/share/doc/HTML/en/kinfocenter/opengl/index.cache.bz2
/usr/share/doc/HTML/en/kinfocenter/opengl/index.docbook
/usr/share/icons/crystalsvg/48x48/apps/kcmopengl.png
/usr/share/icons/mono/scalable/apps/kcmopengl.svgz

none of which is obviously the include files.

So does anyone (a) know definitively what the name of the file the VTK configuration program wants; and (b) what I should download if I don't have it?

Thank you,
Zachary