Eliminate yet another access to the global ConfigData instance.

Eliminate a use of GetGlobalConfig().

release v0.20.21

`$srcdir/.git` doesn't exist if `$srcdir` is a worktree.

This was missing in commit 8266ab55 for #69.

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

Prevent Meson from running Android-x86 binaries.  That will fail
because the Android standard libraries are most likely not installed.

The Curl plugin requires libexpat in order to work these days, so we should
download and build it in order to get the plugin enabled on Windows.

Fixes problems with the Windows build because `-DCURL_STATICLIB` was
missing, causing error messages like:

 "undefined reference to `__imp_curl_slist_free_all'"

first call fchmod() to prevent TOCTTOU, then apply permissions using
chmod()

According to http://wiki.hydrogenaud.io/index.php?title=Cue_sheet FLAC
files should use the "WAVE" file type, but I recently encountered CUE
files declared as "FLAC" which could not be read by MPD.

With a "+" or "-" prefix, the "metadata_to_use" setting can manipulate
the existing default.  This way, one can enable `TAG_COMMENT` without
having to list all the other tags.