media-sound/mpd: Fix mpd build on prefix profiles

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

media-sound/mpd/mpd-0.20.12.ebuild

@@ -226,6 +226,7 @@ src_configure() {
  $(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)
  )