libx11-dev 2:1.6.3-1ubuntu2 (amd64 binary) in ubuntu xenial

 This package provides a client interface to the X Window System, otherwise
 known as 'Xlib'. It provides a complete API for the basic functions of the
 window system.
 .
 This package contains the development headers for the library found in
 libx11-6. Non-developers likely have little use for this package.
 .
 More information about X.Org can be found at:
 <URL:http://www.X.org>
 .
 This module can be found at
 git://anongit.freedesktop.org/git/xorg/lib/libX11