Don't propagate MPD-specific compiler flags to subprojects.

Meson adds "-Wpedantic" in warning_level 3 (which is MPD's default).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1171

Fixes building under GCC 11

That is when meson option b_sanitize is not used

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1168

Fixes -Wformat warnings.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1150

A few exceptions are needed for sources which include COM header
(directly or indirectly).

This fixes lots of shadow warnings, see
https://github.com/MusicPlayerDaemon/MPD/issues/1150

Include the most specific header documented by MSDN instead.

"%lu" is portable - it works with both POSIX and Microsoft flavors.

Fixes a part of https://github.com/MusicPlayerDaemon/MPD/issues/1150

For querying tags, the real song URI should be used, because if the
(display) URI is different, requesting it will not produce a usable
response.  This is a theoretical problem because none of the existing
playlist plugins sets the real_uri.

This requires changing the URI comparison in playlist::TagModified().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1154

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1156

With the default value CURLAUTH_ANY, libcurl needs to probe for
authentication methods first, and only the second request will have an
Authorization header.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1155

Since we migrated to readthedocs.io, we don't need this target
anymore.  And Meson 0.58.0 apparently has a change breaking this
target.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1161