In windowswmproto-1.0.3.tar.gz package, the windowswmproto.pc.in contains following lines: Name: AppleWMProto Description: AppleWM extension headers A guess that's wrong...
Thanks for the report - fixed in git master for the next release: http://cgit.freedesktop.org/xorg/proto/windowswmproto/commit/?id=cc73dda549da6a4fd0a5721315d0dba3f5030fc3
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