This makes ccache more efficient when recompiling with different
plugins.

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

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Aparently, libcdio sometimes returns empty filenames, causing MPD
crashes.  This shouldn't really happen, and I consider this a libcdio
bug - but if it happens, people blame MPD, so let's add a check.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/776

Found with performance-unnecessary-value-param
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-pass-by-value
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-use-override
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Most importantly, this commit translates ZZIP_ENOENT to
std::system_error(ENOENT) so IsFileNotFound() returns true and
find_stream_art() can suppress the log line.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

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!

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Closes #173

Similar to commit 31ab78ae

Fixes deadlock because FileInputStream::Read() unlocks the mutex
(which was not locked) and then locks it, keeping it locked.  This can
result in a deadlock.  This happens because the archive and the file
mutex are different.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Avoid wasting 4 kB stack per directory level.