X11/Xlib.h: No such file or directory

["Martin Welch" <mwelch totalise co uk>]

I'd really appreciate some help. I'm building GNOME from CVS on a vanilla
slackware 7.0, XFree86 4.0.1 box and I'm getting this build error when
building gtk+.

-----

In file included from gdk-pixbuf-xlib-private.h:24,
                 from gdk-pixbuf-xlib.c:24:
gdk-pixbuf-xlib.h:26: X11/Xlib.h: No such file or directory
In file included from gdk-pixbuf-xlib.c:24:
gdk-pixbuf-xlib-private.h:25: X11/Xlib.h: No such file or directory
make[3]: *** [gdk-pixbuf-xlib.lo] Error 1
make[3]: Leaving directory
`/usr/local/src/gnome/base/gtk+/contrib/gdk-pixbuf-xlib'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/usr/local/src/gnome/base/gtk+/contrib'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/usr/local/src/gnome/base/gtk+'
make: *** [all-recursive-am] Error 2

-----

I don't understand this since both /usr/X11R6/include and
/usr/X11R6/include/X11 both exist in my PATH (Xlib.h lives in
/usr/X11R6/include/X11).

What am I doing wrong?

Thanks.

Martin