When the 'systemd' use flag is enabled, media-sound/mpd fails in the configure phase due to the fact that it's looking for a library which no longer exists. All functions previously handled by 'libsystemd-daemon' and others have been merged into 'libsystemd' as of version 209. See: https://github.com/systemd/systemd/blob/master/NEWS#L3596-L3601 for details. For the record I'm using mpd-0.19.14 and systemd-230-r1.
Considering this change in systemd happened 2 years ago (Feb 2014) it's kind of surprising this issue has lingered for so long. While there weren't all that many Gentoo/systemd users then, this bug is likely to become more of an issue with the increasing systemd userbase. If more info is needed, I can be reached on Freenode as either AfroThundr or PityDaFool. Here's hoping this is an easy fix.
For what it's worth, upstream mpd fixed this a while ago (Mar 2015), but it hasn't been released yet: http://git.musicpd.org/cgit/master/mpd.git/commit/?id=400e3ea64960f9bcaf5ab83d7e1bea405bb5fc89
*** Bug 586554 has been marked as a duplicate of this bug. ***
Fixed by version bump to 0.9.15: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=612bbda85991927cd57f5d0f946338768aa54af4
Stable on alpha.
Obsolete for amd64.
(In reply to Tobias Klausmann from comment #5) > Stable on alpha. Disregard.
arm stable
Dear Maintainer (or who is mainly involved in this stable request), This is an auto-generated message that will move the current component to the new component Stabilization. To ensure that the stabilization will proceed correctly, please fill the fields "Atoms to stabilize" and "Runtime testing required" as described here: https://archives.gentoo.org/gentoo-dev/message/4b2ef0e9aa7588224b8ae799c5fe31fa
x86 stable
ppc stable
ppc64 stable
Stable for HPPA. Closing.
