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.

This flag is used by the Android NDK build scripts as well, and this
fixes a build failure (assembler error) with FFmpeg and NDK r21.

Fixes regression from commit db93bb99 because
ParseMimeTypeParameters() assumed the items were null-terminated, but
after that commit, they were not anymore.

The `db->close()` call was a `nullptr` dereference because the `db`
variable had already been moved.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/839

This is the final piece of the series to establish io_uring support on
Linux.

MPD doesn't need io_uring for its efficient bulk I/O support, but to
allow file I/O to be cancelled.  This is a big problem on CIFS/NFS
mounts where processes sleep uninterruptable if the file server
disappears, deadlocking MPD.

With io_uring, a flaky NFS connection allows MPD to continue to work
(even though there are still deadlocks inside MPD which need to be
addressed).

This plugin does not yet use cancellable `open()` using
`IORING_OP_OPENAT`.  This will be implemented later.

Lots of other optimization opportunities for io_uring are still
missing as well - for example the database update could benefit a lot,
but unfortunately, io_uring doesn't have `readdir()` support just yet.

Relax the assertions.  This is necessary if BlockingCall() is used
before the thread is started.