Closes https://github.com/MusicPlayerDaemon/MPD/issues/960

Ugly workaround for https://github.com/mesonbuild/meson/issues/1550

install_man() is currently broken with Meson and doesn't support a
custom target argument.

The problem with this kludge is that both mpd.1 and mpd.conf.5 are
installed in /usr/share/man/man1/, but apparently, there's no solution
yet.

Build with `rst2man mpd.1.rst mpd.1`

This allows automatic optional detection of Sphinx.  This will be
useful when we start building the manpages with Sphinx, which many
users may want to have.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!