where are Xlib.h Xutil.h Xatom.h ecc???

[kkk]

i'm trying to compile a small program (that uses Xforms)
make requests:
X11/Xlib.h
X11/Xutil.h
X11/Xatom.h
X11/keysum.h
X11/Xresource.h
X11/cursorfont.h

where are they placed?
/usr/include/X11 is absent!