libmad recorded wrong pkgconfig cflags which caused configure in mpd to
pick up an additional '-I /usr/include' which then broke because of
mixing up std headers.
Similarly the autoconf check for boost picks up the host system's
/usr/include instead of the prefix's ${EPREFIX}/usr/include, leading to
the same result as with libmad in the mpd build.
An explicity --with-boost heals this.
Closes: https://bugs.gentoo.org/629880
$(use_enable webdav)
$(use_enable faad aac)
$(use_with zeroconf zeroconf avahi)
+ --with-boost="${EPREFIX}"/usr
--with-systemdsystemunitdir=$(systemd_get_systemunitdir)
--with-systemduserunitdir=$(systemd_get_userunitdir)
)