Closes https://github.com/MusicPlayerDaemon/MPD/issues/1130

Remove lots of #ifdefs.

Allows using ranges, always.  This is required to fix
https://github.com/MusicPlayerDaemon/MPD/issues/1130 without adding
more runtime conditionals.

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

The clang/libc++ version in xcode10.3 does not support C++17 properly
and cannot build MPD.

The old "ppa:mstipicevic/ninja-build-1-7-2" just provides ninja 1.7
which is too old and breaks the build.

This enables the JACK output plugin on Windows, but doesn't link
against libjack64.dll, instead loads the DLL at runtime with
LoadLibrary().  This kludge avoids the extremely fragile JACK shared
memory protocol by using the system's libjack64.dll, without requiring
the same DLL at build time.

Fixes crash when pausing the default partition after an output was
moved to another partition.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1147

This reverts commit 1532983f.  This
optimization was bad because now all strings match if they are a
prefix of another string, and this caused collisions in the tag string
pool, corrupting the database.

This is semi-standard and doesn't require the util/Compiler.h header.

This allows using it in constant expressions.

This makes it compatible with containers storing std::unique_ptr.