The xorg-proto-2021.2 tarball is missing the scripts/ directory, so merging fails at the configure step. This issue is known to upstream (https://gitlab.freedesktop.org/xorg/proto/xorgproto/-/issues/31).
Tried https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=8f87481ac571ed2c5f105b746c4872ffd3d2ccfc?
*** Bug 772872 has been marked as a duplicate of this bug. ***