[GtkGLExt] configure: error: X development libraries not found

[patrick hunt <patrickhunt dsl pipex com>]

Hi, I would be grateful for help with the failure of this configure of 
gtkglext-1.0.6 on OS X (10.3.8 + TCSH) as logged below.
X11 and XCode are installed.
Any suggestions?

checking for GNU libc compatible malloc... yes
checking for memset... yes
checking for sqrt... yes
checking for strchr... yes
checking for strrchr... yes
checking for strstr... yes
checking gdk/gdkdisplay.h usability... no
checking gdk/gdkdisplay.h presence... no
checking for gdk/gdkdisplay.h... no
checking gdk/gdkscreen.h usability... no
checking gdk/gdkscreen.h presence... no
checking for gdk/gdkscreen.h... no
configure: GDK doesn't support multihead
checking for gdk_x11_colormap_foreign_new... no
checking for X... no
configure: error: X development libraries not found


Cheers,
Patrick