It is just that the TeX configuration does
not detect it and I wonder why. I even tried explicitely setting
--with-x for configure.
1) Try configure --x-includes=/usr/X11R6/include
--x-libraries=/usr/X11R6/lib to explicitly get the location.
2) Look at config.log afterwards to see why it failed. My wild guess
would be that some special libraries need to be included.