Fixes `-Wpedantic`.

libavutil's macro definition is a compound literal, which is illegal
in C++.  Fixes yet another -Wpedantic warning.

Some more `-Wpedantic` fixups.

Enables `-Wpedantic`.

Fixes regression from commit 69f09648

This branch isn't yet ready for level 3 (`-Wpedantic`) due to several
C++ violations (e.g. variable length arrays).  These are already
cleaned up in the master branch (0.22).

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Closes https://github.com/MusicPlayerDaemon/MPD/issues/866

The WildMidi project added the pkg-config file in version 0.3.3, but
unfortunately, Debian still doesn't ship it 4 years later:

 https://bugs.debian.org/916631

However, for cross-compiling, the pkg-config file is very helpful.

Work around Meson bug https://github.com/mesonbuild/meson/issues/3672

C++ doesn't allow that.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/875

Since we have dropped support for GCC 6 a while ago, we can use
constexpr lambdas now.

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`.

Sync with the Android NDK build scripts.