I'm not sure if this is a bug. x11.pc.in contains a fixed depency to "xau" and "xdmcp". The configure script tells me these dependencies are valid only, if I try to build an Xlib without the xcb low level protocol implementation. At least I can build the X11 lib without "xdmcp" present on my system. Maybe these entries should be generated on demand, if the Xlib really depends on this external libraries.
*** This bug has been marked as a duplicate of bug 15664 ***
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