The protocol documentation says that the difference between `find` and
`search` is that `search` is case insensitive, but that's only half
the truth: `search` also searches for sub strings instead of matching
the whole string.  This part is undocumented and unfortunate, but at
this point, we can't change it.

However leaking this surprising behavior to the new filter expressions
was a bad idea; the "==" operator should never match substrings.  For
people who need that, we should add a new operator.

Prepare to stop using substrings for filter expressions.

Some Boost 1.67 packages apparently have a workaround for the bug, so
let them build MPD.

Without the Vorbis decoder, `libvorbis` is never detected, leading to
linker failures when attempting to build the Vorbis encoder.

Closes #399

Meson 0.47.1 suffers from a bug which breaks linking the MPD
executable because the `-lpthread` flag is not propagated from our
`thread.a`.

See https://github.com/mesonbuild/meson/pull/3895

Closes #403

Fixes linker failure when building without FLAC support.

Closes #400

`HOST_NAME_MAX` is not a portable macro; it is undefined on some
systems.

Closes #402

Fixes #404

Fixes one part of #404

Closes #397

Closes #396

Thanks to C++14, we can declare and fill variables inside `constexpr`
functions.  This means me can stop make assumptions on the `struct`
layouts without losing `constexpr`.

Closes #393

`__linux` is the deprecated non-standard macros which appears to be
not present at all on PowerPC.

Closes #398

Apparently, Meson propagates the linker flags but not the compiler
flags from a `static_library`'s dependencies list.

Closes #392

std::regex_replace() is heavily bloated and overkill for this feature.

Closes #389

expand %h to the system hostname and default to a zeroconf string that
contains %h

Fixes writing to Context.getCacheDir().

All new hardware is arm64 anyway.