Closes https://github.com/MusicPlayerDaemon/MPD/issues/867

Will be needed for End Trimming (RFC7845 4.4,
https://github.com/MusicPlayerDaemon/MPD/issues/867).

Fixes memory leak.  That's what we get for

Fixes the first part of
https://github.com/MusicPlayerDaemon/MPD/issues/867

The formula in osx_output_score_sample_rate() to detect multiples of
the source sample rate was broken: when given a 44.1 kHz input file,
it preferred 16 kHz over 48 kHz, because its `frac_portion(16)=0.75`
is smaller than `frac_portion(48)=0.91`.

That formula, introduced by commit 40a1ebee, looks completely
wrong.  It doesn't do what the code comment pretends it does.

Instead of using that `frac_portion` to calculate a score, this patch
adds to the score only if `frac_portion` is nearly `0` or `1`.  This
means that the factor is nearly integer.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/904

Fixes `-Wpedantic`.

Since Meson 0.51, there are special build options for "native:true"
builds, prefixed with "build.".  This change breaks cross builds
because `GenParseName.cxx` is no longer built with `-std=c++17`.

This patch adds defaults for "build.c_std" and "build.cpp_std".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/890

Commit
https://github.com/mesonbuild/meson/commit/08224dafcba1b694fb624553e7d84deb565aae22
changed Meson to require BOOST_ROOT for cross builds.