Update to use c++_static / c++_shared

[ghost]

It would be nice if you can add support to use the libc++ (llvm) STL runtime. in r9d they're c++_static and c++_shared.

Thanks!

[ghost]

Hey! I would also like to use
APP_STL:=c++static or c++_shared
LLVM seems to offer a full cpp11 support.

Do you have a plan on adding it lately?

[karimnaaji]

Hey, we also wanted to support c++_shared, we're using the toolchain in our project and it's working, it's a really straightforward fix but maybe a start for building something more robust. Main part is here: https://github.com/tangrams/tangram-es/blob/master/toolchains/android.toolchain.cmake#L1031_L1048

[taka-no-me]

Thanks @karimnaaji, your patch looks as a good starting point.