musicpd and c++ requirements

[nia <nia%NetBSD.org@localhost>]

I'd been avoiding updating musicpd because the new version requires
C++17 so needs a compiler newer than the base NetBSD 9 one. since then,
it's been updated and no longer works on armv7 because the pkgsrc version
of gcc8 doesn't work there.

I wonder what the best solution here is, import an old version of musicpd
with a lower c++ standard requirement?

cheers