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!

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

First commit for issue #69

Let FFmpeg do the MP3 decoding.  See commit
a4de9650

Enable the NFS storage plugin on Android.

Closes #226

Closes #213.

Apparently, clang defaults to NEON when ARMv7 is used.  Not all ARMv7
CPUs we target have NEON, so we need to disable that.

Android native code should be position-independent.

The NDK build scripts use "-fpic" instead of "-fPIC" for ARM, but that
doesn't work with FFmpeg's assembly code, because it requires
R_ARM_MOVW_ABS_NC which is unavailable with "-fpic".

By telling clang which implementation to use, we avoid the dependency
on libstdc++.so.

.. and make all library symbols hidden by default.

Saves big amounts of .text section size with --gc-sections, because
only this allows discarding unused functions from those (static)
third-party libraries.

Android is migrating away from GCC, and libstdc++ will disappear
eventually.

Support for GCC has been dropped from NDK r13.  Let's switch
everything to clang before they really remove GCC completely.

For Android NDK r13.

Fixes various static library linker failures ("bad extended name entry
at header"), because the wrong "ranlib" was used.

Reduce dependencies on global variables.