Hello. Headers provided by media-libs/raspberrypi-userland{,-bin} are messed up. This results in configure failures. See upstream bug report in URL. Luckily, Miroslav Šulc fixed this problem in the very recent ebuild for media-libs/raspberrypi-userland. See Gentoo bug #576768. Thus mpv ebuild must adjust raspberrypi-userland dependency accordingly. Also note that so far mpv ebuilds didn't have any kind of dep on media-libs/raspberrypi-userland{,-bin}, which is a shortcoming. Fix is on the way.
Here's the PR that should fix the problem mentioned in the upstream bugreport: https://github.com/gentoo/gentoo/pull/1157
Ok, the aforementioned PR was merged. It needs some testing now.
wscript also states that EGL and GLES2 are required for RPI build. The following PR takes care of it: https://github.com/gentoo/gentoo/pull/1201
The previous PR was merged. mpv-9999 now builds properly on RPI as confirmed by @soredake: https://github.com/mpv-player/mpv/issues/2559#issuecomment-205949890