how to force the caller of configure to pass arguments

[Matthias Langer]

At first, thank you all for your help on my previous problem.
Here is the next one:
How can i force the caller of my configure script to pass certain 
arguments ?
I detail: What i have now is:

AC_ARG_WITH(vtk-headers,
[ --with-vtk-headers The location of the vtk header files ],
[ vtk_headers=$withval ])
VTK_CFLAGS="-I$vtk_headers"
AC_SUBST(VTK_CFLAGS)

However, when the caller of configure forgets to pass this option,
configure will work, but make certainly fail. So it would be great, if i 
can
check if certain arguments were supplied allready in configure and exit with
an error message if not.

Matthias Langer