x11-proto/xextproto-7.1.2 also requires app-text/docbook-xml-dtd:4.5 emerged.
Created attachment 251269 [details, diff] Patch for xextproto-7.1.2 to include docbook-xml-dtd-4.5
I can confirm that with older versions of docbook-xml-dtd xextproto won't emerge.
Confirm bug and solution. Thank you Johan Lidström.
This is fixed upstream by commit https://cgit.freedesktop.org/xorg/proto/xextproto/commit/?id=6aa72ae8e629e065f09a6b6c05504363806d76a3 . This bug should be closed.
marking as fixed per comment #4