Usage of Android v15c NDK with clang requires usage of -c++std c++14

When compiling v5.9.2 using clang with Android ndk 15c it is necessary to enforce c++14, because otherwise clang will fail, see also https://github.com/android-ndk/ndk/issues/421

So though this is not necessarily a Qt issue and it seems to be fixed in the upcoming ndk 16, but maybe you guys want to enforce c++14 or warn for this in the known issues.