minor improvements

[wongsyrone]

Since NDK r16, there are several changes.

1. switch to c++_static

libc++ is out of beta and is now the preferred STL in the NDK. Starting in r17, libc++ is the default STL for CMake and standalone toolchains. If you manually selected a different STL, we strongly encourage you to move to libc++. For more details, see https://android-developers.googleblog.com/2017/09/introducing-android-native-development.html.

2. remove all -O? optimization level

Clang now defaults to -Oz instead of -Os. This should reduce generated code size increases compared to GCC.

It means ndk-build already defines the optimization level, no need to override that.

[wongsyrone]

Just a hint. It's working well on my side.

[Mygod]

Clang now defaults to -Oz instead of -Os.

Source?

[wongsyrone]

https://android.googlesource.com/platform/ndk/+/ndk-release-r16/CHANGELOG.md

[Mygod]

These sound good to me.

@madeye What do you think?

[madeye]

Please feel free to submit a pull request for these changes.