When a `PREBUILT_STATIC_LIBRARY` uses C++ in its sources then the `.a`
file will have a link-time dependency on the C++ runtime libraries.
Android NDK r14 will add a way to give this information to the NDK build
system by adding a `LOCAL_HAS_CPP` setting to the `Android.mk` file.
Add this for exported static libraries that use C++.
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