This allows dropping a few compile-time version checks and we can use
pkg-config to detect the library.

This got lost 8 years ago in commit 87c88fcb

D'oh!

According to
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS_INIT.html
the _INIT variables should be set in the toolchain file.

Right now, zlib is only built for Windows, but we may eventually
changed that, so don't hard-code `win32/Makefile.gcc`.

Make sure all the gory details are visible in CI logs.

For better error logs on CI.

By default, if the parent of a process dies, the process gets SIGHUP
and is supposed to shut down.  This however doesn't work for MPD,
because MPD redefines SIGHUP with a different meaning (like most
daemons do).

To work around this, we configure the kernel to send SIGTERM instead
of SIGHUP.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1706

Fixes a busy loop in BufferingInputStream::RunThreadLocked() because
the method never learns that seeking is ignored, even though the HTTP
stream is already broken and can never be read; nobody cared to check
for errors.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1727

If the environment variable $HOME does not exist, don't attempt to
obtain it from /etc/passwd; without $HOME, the calling process
indicates that it does not wish MPD to access the home directory.

This also prevents MPD from attempting to load
`/root/.config/mpd/mpd.conf` if MPD got started as global systemd
service.  Reading from there makes no sense, only /etc/mpd.conf shall
be used then.

This piece of code was initially added by commit 5d857921.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1687

When drop_start_samples and drop_end_samples overlap and are greater
than the actual number of samples, the `num_samples` calculation in
SubmitPCM() could underflow.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1712

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1721

For CURLINFO_CONTENT_LENGTH_DOWNLOAD_T (commit 4efd0a9f).