Commit 60f957ed broken the GCC 7 build, but instead of working
around missing C++17 features in old compilers, let's update the
compiler version requirements.

This commit raises the clang requirement to version 5 because this is
the first version to support `constexpr` lambdas, to be used to
`Dsd2Pcm.cxx`.

Time to move on, two years after 2017.

According to http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#sections

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!