Google Play refuses uploads with a lower targetSdkVersion.

This flag doesn't appear to work, and since we never had a problem
without the flag, implementing Google's recommendation is useless.

Don't export the symbols of all those static libraries.  Most
importantly, don't export the whole libc++ ABI.

These appear to be no longer necessary (tested with NDK r20b).

Closes https://github.com/android/ndk/issues/951

This implicitly defines __ANDROID_API__, which means we can drop the
"-D__ANDROID_API__=" parameter.

This is recommended on
https://android.googlesource.com/platform/ndk/+/ndk-release-r20/docs/BuildSystemMaintainers.md

It's not mentioned on
https://developer.android.com/ndk/guides/other_build_systems

Using this API function requires SDK level 23.

Fixes nullptr dereference when an exception gets thrown because there
is no ".eh_frame" section for unwinding.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/543

Closes #396

The flag `-stdlib=libc++` doesn't appear to work because it attempts
to load `libc++` and not `libc++_static`, and it omits `libc++abi`.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

MPD has minSdkVersion=21 which is above all the checks here.