10% speedup.  Strange, huh?

For some reason, this speeds up the library by 2%.

By accident, this was only enabled for C.

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

Closes https://github.com/MusicPlayerDaemon/MPD/issues/708

Behave like STL.