error HGraf.c:73:77: + X11/Xlib.h

when i try to install htk
by following this tuto :

http://askubuntu.com/questions/151574/error-while-installing-libx11-devel-package-cannot-be-found

i get error
 error HGraf.c:73:77: + X11/Xlib.h

tosolve i t
i install
sudo apt-get install libx11-dev