android: optional ffmpeg support [cmake autodetect]

build and install ffmpeg for android: (tested macosx)
```
  ./configure \
      --extra-cflags='-march=armv7-a -mfpu=neon -mfloat-abi=softfp' \
      --prefix=$INSTALL_ROOT/android \
      --cross-prefix=$ANDROID_NDK/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86/bin/arm-linux-androideabi- \
      --sysroot=$ANDROID_NDK/platforms/android-9/arch-arm \
      --arch=arm \
      --target-os=linux \
      --enable-neon

  make -j8 install
```
cmake find_package will pick up the installed headers, and libraries, and use it when building for android

CMAKE_PREFIX_PATH should include $INSTALL_ROOT/android