Since linux-headers >= 2.6.38 doesn't ship with v4l1 anymore we need to remove USE="v4l2" from tree and replace it with USE="v4l".
The "new" meaning of USE="v4l" is v4l2 from linux-headers, or libv4l userspace libraries.
v4l - Enable support for video4linux (libv4l or v4l2 from sys-kernel/linux-headers)
v4l2 - Enable support for video4linux (DEPRECATED: Use USE v4l for your package to enable v4l2 or libv4l support)
Bug 359595 (pkgs failing to build against linux-headers >= 2.6.38)
Bug 384733 (stabilization of linux-headers >= 2.6.38)
USE="v4l2" has been removed from Portage. Closing.