Cairo supports many drawing backends, including OpenGL ES 2.0 (gles2), which some have had success getting to work on a Raspberry Pi. However, the ebuild currently expects gles2 support to come from Mesa and has no consideration for Raspberry Pi's VideoCore libraries.
Fix the ebuild such that its aware of Raspberry Pi as an option as an alternative to Mesa/X11, research any patches you may need to use for this, or create a patch yourself with the necessary changes.
Generic global USE variables, such as opengl and gles2, should only pull in X11 when X11 is specified - and should require that a graphical system be specified (aqua and weston being alternatives).
While working on this task you should join and remain in #CopyleftGames on Freenode to get help, feedback, and guidance from mentors and other developers.
When you've done, post your ebuild to this task.