The io_uring buffer is allocated as memlocked memory, as MPD needs to
be able to lock memory.

Before the advent of io_uring (commit dae8da70), this didn't
matter, because the `FileInputStream` never called this.  But
`UringInputStream` is derived from `AsyncInputStream`, and needs the
handler to signal completion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/898

For subprojects.

c++17 is offered by default with Xcode 10
https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Passing `length+1` to `MultiByteToWideChar()` means the function may
fill the whole buffer with output data, and could theoretically
overwrite the null terminator.  In practice, this will never happen,
but this way, it's slightly more correct.

Also, null-terminate after `MultiByteToWideChar()`, after we got the
real output length.  Again, this would never have been a problem, but
who knows...

Needed for std::wmemchr under libcxx
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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).